I attempted installation of SP1 for Windows 7 on an Acer (Aspire Revo 3610) PC running 64-bit Windows 7. The PC runs as a home server and I started the install while remotely connected. Once it completed the install (with about 5 other Windows updates), I allowed it to reboot. 
I eventually noticed that it never came back up (I was unable to ping it) so I went to the console (my TV monitor) and it displayed no signal. The power light was on so I left it alone incase it was just a really long install. I actually installed the SP1 on my desktop, also running 64-bit and though it took a long time (maybe an hour), it did complete and start up OK.
I then powered the Acer off and on and it started a reboot which displayed on the console. It indicated it had a problem and offered a recovery or continue the boot normally. I opted to continue the normal boot. It displayed the bootup graphic and started a count-up of the dlls it was updating similar to my successful install. However, it stopped early and displayed a fatal error:
Fatal Error C0000034 applying update operation (Update 282 of 103814)
I searched the web and did not get many hits on this error and none very helpful other than to recover from a backup or checkpoint.
So, I rebooted again, this time choosing to recover from the last good boot. This also took a while - maybe 1/2 an hour but was successful.
I will reattempt the SP1 install when I can get more information on this error and its resolution.
UPDATE (from this) - Once you are recovered, provide your logs to Microsoft
Go to the folder C:\Windows\Logs\CBS\ and copy all files to your document folder, also copy the setupapi logs from the folder C:\Windows\Inf and the file C:\Windows\winsxs\poqexec.log to your document folder. Zip all files into 1 Zip file and upload the Zip file to your SkyDrive and post a link here: http://social.technet.microsoft.com/Forums/en-US/w7itproui/thread/4fc10639-02db-4665-993a-08d865088d65
UPDATE 2 (A reported solution from thiswoot: here) - Basically, select Launch Startup Repair; follow the steps to get to the command line; run notepad.exe c:\windows\winsxs\pending.xml; reboot and wait about 15 minutes and the SP1 will complete its install.