Now you see it, now you don't -- and now it's back. The very brief tenure of AirPort Extreme Update 2008-003 (it was available on Tuesday for a while, but then withdrawn) has given way to the shining reign of update 004. The update is a 2.2 MB patch recommended for all Intel machines on 10.5.5, resolving "some issues with Airport connections when roaming in large WiFi networks."
Some of the commenters who installed the earlier build didn't experience any obvious ill effects; others reported installation hangs, lockups on reboot or problems connecting to wireless LANs, so your mileage may vary -- with any luck the new build has a steadier grip on stability.
You can download the new update via Software Update or (shortly) from the Apple support downloads page.
Thanks to everyone who sent this in