When you log out of SiteCatalyst you get sent to the version of the server that you were on at the time (sc.omniture.com or sc2.omniture.com, etc). When you try and log in to a different account, you will frequently not be able to (error messages, etc), especially if you are trying to log in to one on a different server.
This is annoying for anyone who uses more than one SiteCatalyst account (consultants!). A simple fix would be logging out would send you to my.omniture.com where you can log in to any server. Alternatively just make it so that if your account is on sc4.omniture.com, you can login via sc2.omniture.com.