I'll post my experience with this.
I just did SP1 on two Dell Servers and an HP DL165 G7.
All with WIn 2008 x64 R2.
Dell's, went about as quick as this update is going to go, which isn't very fast.
The HP got stuck at 15% for about 20 minutes before I went back to my desk to research what might have happened.
While reading this thread and the one at Microsoft reference the Update Readiness tool, I came to understand that the update readiness tool will be automatically be downloaded and run if "it's is determined to be needed. The article I read said that the progress bar will appear to freeze at 60% until it finishes. (Mine froze at 15%)
I had tried to connect via rdp and remote management, and nothing responded.
I burned the Update Readiness tool, printed out pages of documentation on recovering from this, went back down to the datacenter, and the console showed that the server was ready for me to login. (!?)
I logged in and was greeted with a 'SP1 is not installed' popup.
I began the install of the readiness tool, just to see what it would say, and after churning for about 3 minutes it informed me that it wasn't needed and unloaded.
My wild guessing is that:
a.) something had the CPU pegged out to the point where nothing else was responding during the SP install.
b.) Perhaps that 'something' was the downloaded version of the readiness tool checking 'stuff.'
c.) The best bet when installing the SP may be to first reboot, and just be patient. Wait an hour before panicking.
d.) Pre-Run the readniess tool before installing the SP.
Microsofts article on this tool, and the download for all versions of all Vista based (win 7, Win 2008 (r2) etc is here:
http://support.microsoft.com/kb/947821
