So, you're trying to restore a web site backup using cPanel and you get the
Account Creation Status: failed (Sorry, a mysql user with the name blerg already exists.)
Account Restore Failed... Restore failed.
The solution (well, a solution) is to go into phpMyAdmin and check which database has the conflicting user. If you don't want to take the time to change usernames/configs (say, you're trying to restore several sites at once and time is of the essence), simply remove the account with the conflicting username and then restore your site backup. THEN restore the site you removed.
This has worked twice so far. It seems cPanel may sometimes not prefix MySQL usernames properly.