On 12/23 I upgraded our TeamCity server to 6.0. Pretty much immediately our svn server started getting polled excessively with requests that failed authentication. Like 50/s, per our network admin. The solution was to prefix the svn user name with the domain name in each svn root configuration.
The thing is, TeamCity was working just fine without the domain before, and continued to work fine after the upgrade. We only noticed because the admin's monitor sent up a red flag at that excessive auth failures. It's odd that it would fail auth sometimes and not others. The failed auths appeared to be using 'localhost\svnuser' instead of 'ourdomain\svnuser'. It's almost like it was trying it both ways.
We've fixed the problem now. I'm posting this in case anyone else has this issue.