Team Foundation Server is not your current source control plug-in
If you have TFS installed on your machine as your source control provider, and for some reason, it is not being selected as the default source control in Visual Studio everytime you open a solution and it forces you to always pick the source control plugin (from Tools -> Options -> Source Control), it could possibly be due to the following reasons:
1. The existing solution that you have created probably has references to an old source control provider (like VSS) and this could prevent it from resolving.
2. You may have checked in all your projects but you may not have checked in your solution. (TFS does not pick this automatically when you checkin the files). You can quickly verify if this is the case by looking for the lock icon adjacent to the filename in the Solution Explorer. If you do not see a lock icon, right click on the solution and you should see a "Add to Source Control.." option. Doing this will fix the issue.
In some cases in scenario 2, it might complain that binding already exists and you should actually update this via "Change Source Control..". If so, click on the solution, then go to File -> Source Control -> Change Source Control.., specify the TFS server for the underlying projects/solution and this should do the trick.
If it does not allow you to specify the server, then set TFS as source control either by going to Tools -> Options -> Source Control or by clicking on the link that says "TFS is not your default source control". After setting TFS as source control, go to File -> Source Control -> Change Source Control and click on Bind. This will cause a checkout of the files and you will now be able to view the files as sourced under TFS.
If none of these work, you may probably want to look up on this forum
http://social.msdn.microsoft.com/Forums/en/tfsversioncontrol/threads
Good Luck!,
Satish