Mark Russinovich, the founder of SysInternals, and now a Microsoft employee has debunked what we all learned about duplicate SIDS. http://blogs.technet.com/markrussinovich/archive/2009/11/03/3291024.aspx
Note that the advise does not apply to systems that have been joined to a domain and then cloned. Still a really bad idea. Also, SysPrep is still recommended. A worthwhile read.