Hi,
Today I've been struggling with an upgrade of our lab from vSphere 5.0 Update 1 to vSphere 5.1, and I thought I'd share some thoughts and issues I experience during the upgrade.
First some info about our setup. We have a dedicated vCenter (DL360 G4p) with two ESX hosts (DL360 G5) and a shared iSCSI storage. We've started using this equipment with vSphere 4.1, and have been doing inplace upgrades since then.
When starting to set up the SSO component we had some issues due to the installation requiring the "sa" password for our database, which of course we didn't have since we installed 4.1 with standard components. This made us install SQL Management Studio Express so we could set an SA password. Unfortunatly, I couldn't get our SQL 2005 express setup to work with SSO at all, so we had to allow SSO to set up its own SQL 2008 R2 express.
Other than that, the installation went fine. Pretty straight forward, and all thing seems to work like it was intended to 
Now to our production environment, or atleast the equipment we use to administrate our customers.
We have a server in our monitoring domain running the vSphere Web Client 5.0, and we have registrered each of our vCenters here. Those vCenters reside in different AD-domains, and there are no trusts between the domains.
Now, after having upgraded to vSphere Web Client 5.1, we cannot choose which vCenter server we want to log on to (it tries to connect to each vCenter server with the credentials I logged on to the system with), so there is no way for me to administrate all the servers at the same time. However, I can set up SSO domains for those systems, but since they use the same name for that account it will be difficult, and some of the vcenter servers are not part of the domain. Also, it seems to be almost impossible to add another vCenter 5.1 server to the vSphere Web Client, all I get is reference to the "lookup service", but no instructions on how to use it to add another server.
Any suggestions on how to solve the vSphere Web Client issue are welcome, I'm near to submit a support case for this.. 