Ive got 2 access points on the same ESSID, same WEP, and same channel and I roam fine.
Of course dave's solution of 3 channels apart probably would be better.

APs bridging or NAT+DHCP?

An "AP" that is doing NAT+DHCP is a router (with AP function), not just an AP. An AP is just a switch that is translating between one form of Ethernet (wire, or 802.3) and another (wireless, or 802.11). It doesn't even need to have its own IP address, although they commonly do for management purposes. It's a layer-2 device otherwise. A router/AP combo unit is a layer-3 device.

802.11 channels are orthogonal at 5 channels apart, not 3. Which is why nearly everyone uses channels 1, 6, and 11, only.

Your setup need not be so complicated. What you want is a router and two APs. One of the APs may be a combo router/AP. Just connect the second AP to a LAN port of the router/AP unit. Give both AP's the same ESSID and crypto settings, but different channels (to minimize interference). You will roam between them based on signal strength.

My main access point (wrt54g) is upstairs and I have a repeater (airlink 101) downstairs, both are set the the same SSID and I can roam between them without dropping.

FWIW, My roommate and I did this (he has the laptop). And his laptop would drop out on a consistent basis, like, every couple of minutes.
So far as we could tell, it's when the wireless card was switching communications.
I don't believe we did the 'at least 3 channels apart' thing, but it could very well be a specific card design / driver issue.

YMMV

I have 2 (sometimes 3) WRT54G's wds'd together, same ESSID, keys, and channel. I had no idea until reading the other comments that you could do that without wds.

P.S. dd-wrt >>> hyperwrt, but I'd rather have an old computer running OpenBSD than either of them :P

Had never heard of WDS until this. Reading briefly, it doesn't appear to be what I want:

1) the two APs couldn't talk to each other, since concrete wall is between them.
2) I have Ethernet everywhere anyway.

Unless WDS can also allow all nodes to have Ethernet, and do backchannel stuff via that.... but from what others have said, I might not need any of that.

Ah, I'm totally wrong. WDS is not solving your problem.

Some Linksys firmware versions have an option to forward DHCP requests to another IP, so I think you could run it in a standard configuration using that.

dd-wrt >>> hyperwrt

I should hope that DD-WRT improved since my last tries with it as it went braindead once I started to use P2P software. That's why I switched to HyperWRT ;) (Hyperwrt 2.1b1 + Thibor15c)

i have 2 APs, identical wifi config except for channel number, and roaming between them appears to be seamless.

I think there's a proprietary Cisco standard for doing AP migration but I hear it doesn't work well. It was on the Seattle Wireless mailing list a few weeks ago.

But if both your APs are on the same Ethernet segment and have the same SSID (and/or WEP key) your wireless card should reassociate freely between the two and your IP traffic will only see minimal distrubances as you move from one AP to another while your switch ARP tables retrain. You'll definitely NOT want the APs to be on the same channel as they are not be designed to cooperate with each other and even if you make a reliable connection, RF crosstalk >2 clients will destroy your performance.

Of course your APs will have to be in bridge mode which probably means you'd have put them together on a switch with your linux box and have linux route/filter/bridge the whole wireless subnet.

I'd just do the bridge thing. However, jut be aware, not all 802.11b channels are orthogonal.

http://en.wikipedia.org/wiki/IEEE_802.11

Since the spectral mask only defines power output restrictions up to ±22 MHz from the center frequency, it is often assumed that the energy of the channel extends no further than these limits. In reality, if the transmitter is sufficiently powerful, the signal can be quite strong even beyond the ±22 MHz point. Therefore, it is a misconception that channels 1, 6, and 11 do not overlap. It is more correct to say that, given the separation between channels 1, 6, and 11, the signal on any channel should be sufficiently attenuated to minimally interfere with a transmitter on any other channel. However, this is not universally true; for example, a powerful transmitter on channel 1 can easily overwhelm a weaker transmitter on channel 6. In one lab test, throughput on a file transfer on channel 11 decreased slightly when a similar transfer began on channel 1, indicating that even channels 1 and 11 can interfere with each other to some extent.

Although the statement that channels 1, 6, and 11 are "non-overlapping" is incomplete, the 1-6-11 guideline has merit. If transmitters are closer together than channels 1, 6, and 11 (for example, 1, 4, 7, and 10), overlap between the channels will probably cause unacceptable degradation of signal quality and throughput.

So, use channels 1, 6, 11.

Thanks!

and WPA auth

finally, one smart voice... Brad, please, no WEP - it is same, as you were hiding frontdoor key under carpet.

Two WRTG running DD-wrt (not that it matters). Have one do DHCP for you, and set them both to the same ESSID. Everything just works :)

The AP's in question don't technically even need to be the same ESSID, the only thing that matters they forward their "control" bits between each AP.. this is called IAPP; most AP's do this in the form of some UDP packets. Avoid WDS unless you like your bandwidth cut in half.

Yeah, IAPP is required if you want to roam authenticated access (e.g. 802.1x) effectively. If you don't have it, the client has to reauthenticate every time it switches AP, which doesn't work when you're backing to RADIUS. I don't think that's an issue, however, for WEP/WPA + DHCP.

a big noisy Linux box

Brad, you seems to live in stone age - Penguins can be not that big and noisy

My main snowdrop AirPort base station, which ties into the DSL line, sits in my room. My AirPort Express, which gets used more often, sits in the living room, and is set to "join [the] existing wireless network" created by the snowdrop: computers in the living room receive their signal from the Express, and I know that because the light flickers a lot.

Are you against damaging that wall? If you want to play with power tools you can go low tech and plumb a small hole through it, so you can put the AP on one side and using either an new omni antenna or an extension cable to move the OEM entenna into the other room to give coverage where.

(I am assuming your AP has 2 or more antennas and has TNC/SMA or some other threaded antenna fitting. Basically, a Linksys.)

I believe it's a load-bearing wall. I'm against damaging it. :-)

If both APs are backed to ethernet directly, you don't want WDS. All you need to do is set both APs to the same ESSID and your wireless card and in dumb bridge mode and the clients should roam automatically without mishap.