I may have discover a solution to this. I was faced with this issue and out of pure frustration, I finally deleted all of my plugins with the exception of the default ones included in the distribution package of WordPress, and found that I indeed had 1 that was not compatible with WordPress 2.5.1.
The WP-SpamFree Plugin did not like the changes in 2.5.1 for some reason. If you have this plugin installed, try deactivating it, or deleting it and recheck your results. There may also be another in the group of installed plugins causing the issue.
I am the author of the Gaboinked! Chipin Sidebar Widget for WordPress and can assure you that it is compatible with WordPress 2.5.1.