I find this *really* convenient...
Bring up someone's blog.
Add a comment.
Log in using your old (pre-beta) acct info.
Put in the word-verification string.
Machine tells you your acct has been merged with Google acct such-and-such, gives you a button to log in with Google acct.
Log in with Google acct.
You're no longer anywhere near the blog you were on, and your comment has disappeared.
Well done, guys!