Not that it is a hard find, butwe had a problem crop up yesterday with a wordpress mu installation that was causing a permission error when media was attempting to be uploaded to the blog. Here is a nifty solution which seems to be a common issue (link to thread):
This worked for me, using a Plesk environment:
Make sure the web server user (apache in my case) is a member of the group owner of the blogs.dir folder (psacln in my case.) Doing this enabled me to keep the blogs.dir and subfolders 775.
usermod -G psacln apache
We were also working under Plesk – so it was a double bonus.