This might have been obvious to everyone else, but it wasn't to me: although the AirPort Extreme base station can talk to 802.11b and 802.11g clients, it looks like the AirPort Extreme card can only talk to 802.11g base stations.
[Update: I was wrong. Two things fooled me. First, and silliest, is that networks that aren't broadcasting their ids don't show up in the list; that's what "Other..." is for. I knew that. Second, and most surprising, is that people in my building seem to turn their base stations off! I don't often look at the list, because I don't usually need to, but it turns out to have been pure coincidence that the 802.11b networks I was familiar with from my old PowerBook's menu weren't available. Thanks to Geoff and Nathan for convincing me to try harder!]