I had two problems with WordPress since I updated to 2.5.1 last week: the trackback field disappeared and no one could leave comments on the posts. The issue turned out to be an incomplete upload of the wp-admin/inludes folder. Once I re-uploaded the folder, the trackback field and comments-enabled options re-appeared on the Write New Post page.
If you encounter any posts that do not have comments enabled, please let me know. I’ll have to verify that things are still working okay. After a database conversion to 2.5.1 and having to re-upload the inludes folder, I wouldn’t be surprised if something else needed to be tweaked.