I have always religiously run SysInternals' NEWSID when I cloned or imaged machines. I recently learned that doesn't work on well on Vista/Windows 2008 and doesn't work at all for Windows 2008 R2 or Windows 7.
I am shocked now to learn that this has not been necessary all along. I'm still digesting it all, but Mr. SysInternals himself says this isn't even necessary and NEWSID is being retired.
In this eye-opening article he walks through every use of the machine SID and proves that a duplicate machine SID in a workgroup or domain will not cause any problems.