I thought I would share my experience going from 5.1 to 5.5 using the guide from above:
I tried doing an in-place upgrade of my 2008 R2 vCenter server (connected to seperate SQL 2012 VCDB database servers and no SSL Certs), and the SSO prerequisite check steps bombed out because apparently SSO doesn't sync passwords in "Linked-Mode". Plus, the complex passwords I used in the SSO GUI, don't play nice behind the scenes if you have special characters in them. (SSO accepts the password anyways I guess in 5.1).
So I followed the guide above, and all is well, until I find out that when you move the host into the new 5.5 vCenter server (using the existing VCDB database), the hosts can't synchronize their dvSwitches. (If someone can shed some light on why I haven't heard a peep about "Exporting your dvswitches via the web client", that would be great.)
Plus, VMware needs to overhaul their "Alert Management" console window. It's a very painful experience when you have to re-configure rules, with no ability to export, import, backup, restore, etc. (There's no official way to copy alerts among vCenter servers???)
Besides those two items, it worked out pretty well. Thanks for the guidance, and I hope SSO goes better from here on out.