Again, what are you using as an AD domain controller, or probably more importantly what is the domain functional level? It's possible that some group policies especially those that are a product of 2k or 2k3 AD, are not being interpreted properly by the Win7 machines. Perhaps taking a look at the local event viewer on a machine that is having the issues may reveal some additional information, filter for event errors with a source of userenv.
How do you typically recover from the problem, can the issue be fixed by logging off and logging back on or a reboot, or does it persist for a or those specific users or even a or those specific machines.