I was having issues updating WordPress from Version 2.9.0 to 2.9.1 via the core update functionality installed within WordPress’ admin.

I had a plug-in installed called Web Invoicing and Billing after I deactivated this plug-in, I was able to upgrade to the latest version of WordPress Successfully.

If you are having issues upgrading, try deactivate some plug-ins – 1 at a time, before you resort to a manual install.