As near as I can remember the GUID in SQL Server is based off of the MAC address of the NIC and the current system time so yes the GUID *should* always be unique. (I believe there is some mathmatical inprobablity whereby you can wind up with a duplicate guid but the chances are so low it isn't worth worrying about)
To this end I see nothing wrong with the structure you have shown in your post, in fact I am currently working in a system where we employ this very same technique and have not had an issue with it thus far.
hth.
-Doug
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