[link|http://wiki.wordpress.org/?pagename=ReleaseNotes|This] says it's working, but the installation instructions and requirements don't mention it. There's a Debian package for v1.2.2, which should include the Postgres backend, but the package requires you install MySQL (which I'd rather not). Guess I'll download and install from tarball and try it out.
[edit] Well damn. That page above says that PHPWiki 1.2 has Postgres support, not WordPress 1.2.