There are two primary database changes made in version 3.0...
1. Two tables which are used for keeping track of OpenID transaction data (associations and nonces) are removed, and this data is stored in the WordPress options table instead. If you were to rollback to 2.x, the old tables would be automatically rebuilt and everything would be fine.
2. Versions 2.x tracks which comments were left with an OpenID by adding an 'openid' column to the comments table. Version 3.0 removes this column, and copies all the data into the posts_meta table. If you were to rollback to 2.x, the column would be recreated and used for future OpenID-verified comments. However, previous comments that had been left with an OpenID would not display the OpenID logo next to the comment. The comment itself is entirely intact, it's just that the flag saying it's an OpenID is gone. Of course the data still exists, it's just in the post_meta table instead of the comments table. You could write a script to pretty easily copy the data back over.
User registrations and such should work without any problem. Some the options you set for the plugin may be missing because they were renamed, but those are pretty easily fixed. And of course, backup your database first! :-P