Keep in mind that at Microsoft itself, Vista clients coupled with 2k3 SP2 servers is an extremely common configuration, perhaps the majority situation by now. If what you described were true for every vista client coupled with 2k3 servers, most of the campus would be having the problem. So I'm guessing the issue is something slightly more complicated than that.

I'm glad you found a good work-around with that netsh command. Was that in a KB article somewhere that you found it?

I don't believe 2k3 SP2 did any major overhaul of the network stack. I'm pretty sure it was only minor bugfixes.

Frequently these kinds of things can be traced down to problems with the firmware on routers and switches, and the problem is usually very subtle and hard to track down. If it only happened after the SP2 upgrade, it's probably just a tiny little thing that set off some subtle bug somewhere.