I'm having the same problem with XAMPP 0.7.2 on Mac.
All MySQL fields that WordPress uses with auto_increment are generating huge numbers.
Like the original poster of this thread, I cannot create valid new posts, pages or categories in wordpress (actually they are in the database, but wordpress does not seem to think they are valid).
I tried installing and reinstalling XAMPP and WordPress on two machines, one with Leopard (10.5.2) and one with Tiger (10.4.11). Same issue.
BTW, I tried MAMP with Wordpress (It installs an older version of MySQL) and everything works fine.
Any info on this is appreciated.