I have installed SBS2011 on Proliant 350 G6 with 20GB RAM.
When I shut down the system it takes a little over an hour.
Suspecting a RAM problem, I have tried to boot/shutdown with various RAM configuration (including alternative DIMMs) and I have found that as long as I keep the memory under 4GB the system will shut down in 10 minutes. Anything above it and the shut down time goes to 1 hour.
Other than the shut down time every thing else seems to be working OK without any errors.