Internet Sharing supports WEP but not WPA?
It is not possible to use OS X's internet sharing feature to create a WPA- or WPA2-protected network [through v10.7 Lion, see below]; WEP (40- or 128-bit) or no encryption at all are the only available options.

Note that the original title on this question ("Airport supports WEP but not WPA?") was a bit misleading, as this is only a limitation on wireless networks created with internet sharing.  Mac computers can join WPA and WPA2 wireless networks, and have been able to for years and years.  Similarly, Apple's Airport base stations (except for early models) can create WPA and WPA2 networks.  It's only when the Mac is acting as a base station that this limitation exists.

UPDATE: In OS X 10.8 (Mountain Lion), the normally-visible security options changed to WPA2 Personal or no encryption. WEP is still available in 10.8, but only if you hold Option while clicking on the Security pop-up menu.