Just found a bug in RC 1.
If the Comment Moderation list under Options->Discussion is blank, then when a returning user comments on an article strpos on line 867 of comment-functions.php will fail. Adding a word to the Comment Moderation fixes this.
The line should be modified to take in account a possible blank setting.
See:
http://wordpress.org/support/topic/51095#post-280388
for the errors themselves.