Having the same problem.....Apple must have changed something on their end then. Unable to get any of the latest downloads like iPhoto 9.3, iTunes 10.6.3, ARD Admin 3.5.2, Java for Mac OS X 10.6.Update 9, iMovie Update 9.0.6, etc.. Last update I got was Digital Camera Raw Compatibility Update 3.13 on 5/22/12.
We aren't the only ones.......about 10 posts or so on Macenterprise.org mailing list about this issue. I can't beleive Apple did this and took this many SUS servers out, hopefully it gets fixed and very soon!
Just seen this other thread....people are saying Apple knows about it and are investigating it.