You're right there with SSO, it has been the single piece of vCenter 5.1 so far that I've had to touch more than anything else outside of getting in and building the heirarchy and roles in vCenter.
Good call out regarding multi-site SSO, I made sure I caught that gem of information before actually installing. At least even with a manual process, VMware gave us the import/export utility.
I might just end up waiting until the network is set up to allow cross-datacenter communication for vCenter before doign it. It will just delay this migration project by 1-2 weeks which I can afford since I'm now ahead of schedule (woohoo).