Just upgraded to WordPress 2.9 Release Candidate. Some time later, I realised my blog had a fatal error about the_post_image function being undefined! After some Google and Twitter search (one of the thing I love about Twitter), I found out that the new thumbnail feature has different function names on WordPress 2.9 RC than the Beta 2!
If you are having the same error in WordPress 2.9 RC, note that the development team has decided to change the function names:
the_post_image has been changed to the_post_thumbnail
has_post_image has been changed to has_post_thumbnail
It is a better naming in a way although I’m sure the upgrade has made many WordPress bloggers pulling their hair out in frustration. A nice big warning about this change should have been written on the WordPress 2.9 RC release blog post!