I know Intel included support for it, but I haven't seen anything official from Apple. Nor on the ability to use 1333MHz memory with the X5550 & X5560.
However, my instincts tell me the 32GB option would be Registered, and 1066 was selected for cost & a unified parts sustem.
I think so too. I mean I don't see any 4GB ECC UDIMMs and that ASUS board that was linked to was 96GB RDIMM and 24GB UDIMM, but then you can get 4GB non-ECC UDIMMs.
You'd probably want registered with those sizes anyway.