I just read a brilliant blog post on Carnal0wnage.  Luke Jennings' Incognito has been integrated into metasploit.  Check out Luke's recent paper if you missed it.

In a nutshell, if you get SYSTEM level access to a box (e.g. MSSQL database) and a domain user is logged into that box, then you can use meterpreter (or the original incongnito tools if your prefer) to impersonate that user.   Equivalently, if you want to be a domain admin then find out where he is logged in, and if you can own that box, then you can become a domain admin.

CG walks you through exactly how to use new features of incognito.  You might also want to check out his first post on this topic too.