Since we are able create and use x64 VMs from within vCenter/ESX, you would expect everything to work regardless of the architecture. Now, I'm not claiming to have a deep understanding of how View Agent and Composer work here. However, since they are both required components for linked cloning and neither support x64 OSes, I am assuming there is some sort of incompatbility.
I realize I haven't given you a concrete answer as to why this doesn't work (and I might not I have one), but I'll throw out some other obvservations I've made. I already mentioned the View Agent USB driver doesn't work on x64. When testing linked clones on x64 OSes, I noticed I couldn't select any of the x64 desktops with snapshots. Out of curiosity, I created an empty Windows XP Professional x32 VM (with no OS), and took a snapshot. I then walked through the steps to created an automated pool, and was able to pickup the snapshot for the empty x32 XP VM. This told me that there are no checks on the actual OS for linked clones through composer. They must be performing a check on the stated "Guest OS" when you create the VM, and only allow x32 VMs to be used as parents for linked clones. I'm not sure if there is a difference in the virtual hardware in x32 XP vs x64 XP, but it may be worth creating a "Windows XP Professional (32-bit)" VM and installing XP (or Win 7) x64 just to see what happens.
If I get a chance to test this, I will post back. I'd be interested to hear what anyone else says, since I am very interested in getting x64 support as well.
