Sadly, I'm not sure that that's quite true - one of the things that doesn't seem to be sorted by sysprep is the WSUS ID - hence all the suggestions in this thread for deleting the WSUS info.
Originally Posted by p858snake
What I do is make the image, run sysprep and shutdown. I then boot the machine using Windows PE and delete the registry keys mentioned (you need to load the hive c:\windows\system32\config\software to get at them - there's no point deleting stuff from the WinPE registry!)
I also delete the contents of c:\windows\softwaredistribution - this cleans out all the software used by Windows update but when the service starts in the newly deployed image it will find this is missing and fetch a new copy from the server.
This time in WinPE is also good just to clean up - remove any temp files which don't need to be there, any log files in C:\windows (windowsupdate.log is a good one; you then know that anything in it is actually relevant to your newly booted image but I think every log file in c:\windows can go)
You can also delete pagefile.sys and hiberfil.sys (if it's there). On a machine with 4Gb of RAM these 2 files can add 8Gb to your image size (although you don't need to worry if you're using ImageX - it doesn't include those files)