Thanks for the feedback! Unlike me, I take it you did this upgrade more than a week ago?
I'm kind of suspecting my test server freaked because of a VMware issue. We've been experiencing a problem whereby random VMs that have static IPs show a 169.xxx... address in vSphere where VMware KB articles pointed to a problem in an older ESX version. However, the host that my VM is on is running the latest 5.5 release so not sure what's going on. Bouncing the VM's NIC connection in the OS usually does the trick but never has CPU usage been so high that I can't really do anything in Windows.
Anyway, knock on wood, my server's still behaving itself this morning where I noticed yesterday that SIM is now reporting the availability of HF4. Have yet to check its release notes but thinking maybe I should just chance it on my test server after what happened to it. Anyone take the HF4 plunge yet?