Rehashing again, yes, the original Airport card can do WPA. (The important difference between WPA and WPA2 is that WPA uses "TKIP", which leverages the same encryption hardware as WEP but rotates keys with every packet, thereby providing a workaround for the fact that WEP itself is easily breakable once you've collected enough packets. With the right software any card that can do WEP should be able to do WPA; I had an ancient Wavelan Silver card set up on a Debian box to be able to connect to WPA networks, although to be fair doing so requires softloading a custom firmware. WPA2 requires AES encryption capabilities which pretty much all 802.11b and some 802.11g cards lack.) There should be a checkbox somewhere on the Airport Extreme's UI to have it allow WPA in addition to WPA2 and it *should* work, although I will grant I haven't tried a WPA-only client on my Airport Extreme since the last successful firmware upgrade.

(I mention "firmware upgrade" because I did actually have some issues a couple versions back with an upgrade mysteriously breaking every non-Mac client in the house, which apparently only happened to me. If you end up really stumped I might suggest blowing away the config on your Airport entirely and re-setting it up from scratch, which is what I ended up doing to fix it.)

Allowing WPA does in theory open up a couple edge-case DOS attacks that don't work on WPA2, but I seriously doubt anyone would waste their time on them in a residential setting.