Another option you have, and the way I just did it, is to keep running your vCenter 5.0. Run the install for SSO from the 5.1 ISO and get that working first. It doesn't interact with 5.0, so you shouldn't mess anything up. Once you have that working, you can install the WebClient service too, and also get that running - it interacts with SSO, so you can do some testing and make sure it's all happy. Once all that's done, pick your time, and then do the rest of the vCenter 5.1 upgrade.
I guess ours was a reasonably simple setup - we run all the vCenter services on the same Windows server, and we were already running SQL 2008, so there wasn't too much messing around to do - this was with 13 ESXi5.0 and 9 ESXi4.1 hosts, and around 180 guests.
The SSO/Web Client Service took me a couple of hours to do back in February, and the final part of the vCenter upgrade took a couple of hours of down time to do last week. Of course, you can add on another 2+ hours for installing syslog, dumper, autodeploy, vShield and other pieces that you have.
My biggest fear was all the horror stories I had read about the SSO install, which is why I wanted to get it done and out of the way first.
Of course, I am still building and testing my ESXi5.1 ISO install image etc.
G
