Just upgraded to 2.1 and been doing some testing.
I use Postmeta fields extensively on my blog, so I was pretty flustered when I found that when I added a new comment to a post, ALL of the Postmeta fields were being deleted.
I checked the DB PostMeta Table, and discovered that yes, all PostMeta entries in relation to that Post ID had been dropped from the database.
1. Make a comment
2. Approve comment for posting on site
3. Check your Postmeta fields (and discover they are all gone).
4. Check the DB Postmeta Table, and find that all rows for that PostID have been been dropped.
Further info: I upgraded from 2.0.7
I'm running on Dreamhost, using PHP 5.1 (Im pretty sure), and MYSQL 5.0.