DON'T DO IT!
This is my advice if you want to upgrade from vCenter 5.0 to 5.1 - forget it!
We have just spent two days trying to get SSO to allow us logon through AD accounts and it just won't work for us.
In the process we have scrapped the original vcenter 5.0 install and tried a fresh install of 5.1 but it still it won't work. We are now putting vcenter 5.0 back on after a wasted two days.
We are a tiny customer we use vSphere business essentials, and it's great, really good value for money and it's generally a joy to work with. However, I can't recommend the upgrade to 5.1.
I can see the user accounts from my AD fine but I can't login with any of them. I have added users using the web client as stated in lots of blog posts but nothing works.
I know I'm sounding off a bit but that's was a real frustrating exercise and wasted two days of my time!!!!
I'll be wiating for 5.2 now before we try this again.