Just FYI, if you are upgrading to the brand new WordPress 2.5 release and you are using the K2 theme, expect some problems. Apparently if you are using K2 RC3 or earlier, you’ll get the following error:
Call to undefined function wp_register_sidebar_widget
…and will be unable to log in to the admin interface. At least that’s what happened to me and quite a few others.
According to the K2 site, they should have full WordPress 2.5 support soon. In the meantime, I’ve found that the K2 RC4 release works fine.