Its not the last build. Microsoft plans to release the final Serviec Pack 3 in the midle of 2008.
The 5503 is interestiing cuz of jump from 3311 to 5503. It is sayed that 5503 build has an improved security (very improved), but the Microsoft doesnt aprove or denny that this build is official so it its just to guess what MS is planning.
A lot of sites says "Use at your own risk."
EDIT:
By the way has anyone tried to install it in VMware and get the Cannot format drive, it might be corrupt... ?
tried many times integrated with nlite (not working), integrated throught command line (not working), , with some addons(not working). or maybe i have a corrupt setup but when extracting it doesnt show error.