For those of you that have attended recent Technet Briefings and picked up the complimentary copy of Vista Ultimate, you will notice that we only provide the 32bit media for Vista. However, if you can get your hands on the 64 bit media (ie; borrow it from a friend), the key you receive after you get registered online will work and is valid to use with 64 bit media.
Or to be more specific, the key works with Windows Vista Ultimate x64 SP1 media.
Good Catch! You must use the Vista x64 with SP1 integreated.