On the requirements page at http://wordpress.org/about/requirements/ it says "PHP version 4.2 or greater." This is no longer true for the new 2.3 release. I believe that php 5 is required (the code uses stripos, which is a php 5-only function according to php docs).
I'm glad I backed up my files before installing 2.3! It would have saved a newbie some time if that requirements page had been correct.
By the way, this window I am now typing in to make this post asks "What version of WordPress are you using?" and does not offer "2.3" as a choice.