Well the size is half the size of the GUID.

I'm not sure how GUIDs are generated but I'm willing to be there is substantially more calculation involved albeit highly efficient and optimised calculation.

As to not being able to come up with something better than GUIDs that implies that GUIDs are at the pinacle of identity generation and can never be surpassed.

I have heard anecdotal evidence that GUIDS are only guaranteed unique on the machine on which they are issued. It is improbable that you will get two systems generating contradictory GUIDs but not impossible for it to occur.

Just as a matter of interest how do you handle multiple data centres, particularly where there is a large geographic distance between them?