I should have waited a day. Parallels released a beta version of Parallels Workstation for Mac OS X today for free download.
Naturally, I had to install it right away, too. It's defintely a beta version, and has some serious bugs, the biggest of which is an OS X kernel panic if you use Ethernet networking (Airport wireless networking is fine), but it does run Windows XP just fine.
It's nice and inexpensive, too: the registered version will be only $49 when it's released. If they get the bugs straightened out, I'll probably buy it.