I've been searching for a solution to this problem for hours. Everyone seems to be experiencing a problem when visitors try to comment on a post, and they get a 404 Page not found error. This was happening for me on all my sites with WP 2.7.1.
I've seen people in forums talking about resetting permalinks, adding code hacks to files, reinstalling WP, disabling plugins, etc. None of that seems to be working for anyone. I figured it out!
After staring at code for hours, I decided to look at the files on the server with my FTP software (WS_FTP Pro). I discovered that the file "wp-comments-posts.php" did not have any file attributes (CHMOD values).
Furthermore, I could not CHMOD the properties on the file... so I deleted it and uploaded a fresh one to replace it. The new copy automatically had security attributes of 644 (RW-R-R).
I reloaded my site in a browser, completed the comment form, and BINGO, it works!
I don't know why the file lost its CHMOD properties on the server side, but deleting and reloading it through my FTP program did the trick.
Comments now work on all my WP sites.