I found that UAC really interferes with things on W7 far more than VISTA or other OSs.
I've got a few in each group that won't upgrade, no rhyme nor reason, in the server group, 8 of 10 did, 2 did not, yet I can push the install to them just fine.
So for some reason, with this release, some clients don't do the upgrade, while most do.
With the W7 machines, only those with UAC shut down upgrade successfully.