Jeff Chandler, at the Weblog Tools Collection blog, posts that WordPress 2.9.1 beta version will be available shortly to address a WordPress core bug in the 2.9 version.
While undoubtedly version 2.9.1 will include additional bug fixes, there is a bug fix reported in WordPress’ Trac issue tracker:
http://core.trac.wordpress.org/attachment/ticket/11505/ticket-11505-full.patch
So those of you who wish to assemble your own bug fix instead of rolling back to your pre-installation backup can give it a try. 
Note: Since this is PHP code, it would be helpful to be familiar with PHP.
