2009/5/20 MinuteElectron minuteelectron@googlemail.com:
You can find the revision number at http://en.wikipedia.org/wiki/Special:Version and simply check out trunk at that revision.
The problem is that that omits live hacks and individual files that have been updated. It might leave out some critical security updates, for instance, which are often applied to Wikimedia long before an actual scap.
It's reasonably reliable, though, if you don't mind possibly being vulnerable to XSS or SQL injection from time to time.