Ran in to a situation a couple of days ago where i needed to log into a Team Foundation Server as a different user - and since Visual Studio/Team Explorer "remembers" your last credentials you don't have a chance to re-enter these credentials. This information is cached even after removing and re-adding the server in team explorer. Which begs the question - where is it cached?
Turns out we are relying on Windows to do this for us. To dump this cache all you need to do is go to control panel > User Accounts > Manage Your Network Passwords select the Team foundation Server and choose remove - viola! Next time you go into Team Explorer you will be prompted for a new set of credentials.