The solution you have opened is under source control but not currently configured for integrated source control in Visual Studio

One of our developers receives the following dialog every time he opens a solution from the Source Control explorer:

This behavior is apparently an "issue" in Visual Studio 2008, that will be fixed in the first Service Pack. This happens when the Team Foundation Server instance is offline. When you open an "online" solution it will be put in "offline" mode. The real issue is that Visual Studio should detect when the Team Foundation Server instances comes back "online" to put the solution back "online". As a workaround you need to take the solution "online" manually:

The advantages of this functionality is that you can work offline on the source code and when the TFS server is back up and running you take your solution "online" and it automatically tracks the changes.