I have a post-commit hook that cds into my website's directory and runs svn update. For my serveradmin, it works correctly. For other users, the commit works and the hook executes, but the update returns "Skipped '.'" to the log file.
Here's as much pertinent information as I can think of:
Tying up this old question since, as I mentioned in my comment before, this ended up being a limitation with Media Temple and their support team confirmed that there is no workaround.