Anyone who tried upgrading to vCenter 5.1 probably got smashed by certificate issues.
If you're like me you relied upon Derek Seaman's 14 part blog to get me into a working state.
http://derek858.blogspot.com.au/2012/09/vmware-vcenter-51-installation-part-1.html
In addition to this VMware have used some of Derek's steps to create an official KB article on the subject.
My point is that VMware needs to give Derek an award of some sort, he saved the day for thousands of people out there and helpd VMware get past a particularly dodgy release of vCenter. Even though he is a vExpert, he needs an even higher award than that.
That's just my two cents worth!