http://www.woodwardweb.com/vsts/000286.html
Hack #1 - Stopping Team Explorer from connecting on Visual Studio 2005 Start-up
Add a DWORD value called "AutoLoadServer" under HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\8.0\TeamFoundation. Zero means do not reconnect automatically.
I first read about this setting over at Tim Noonan's blog. This can be very handy if, like me, you frequently change TFS servers. I jump between the work TFS instance, CodePlex and a test or VPC versions of TFS. I know which server I want to connect to when starting VS 2005, and frequently it is not the one I connected to last time. Adding the following setting means that I have to manually pick the server I want to connect to from the "Add Existing Project..." button.