When TFS 2005 was released there was no offline support, if you went somewhere to which you had no connection to TFS you couldn’t be considered an happy camper.
After a few months, Microsoft released a version of the Power Tool that had the ability to work offline. But it was a very manual process. You had to remove the read only attribute of a file and when you got your connectivity back, you would invoke the (command line) Power Tool command so changes could be reconciled (files without the read only attribute would be checked out, new files would be added, etc.).
With the 2008 release things got a little better. Offline support was added. If you opened a solution which was binded to TFS source control and you didn’t had network access to TFS Visual Studio would be asked if you wanted to work offline. (Most) changes would be recorded and when you went online changes would be reconciled (much like the power tool capability but integrated with VS and the GUI).
It wasn’t much better than 2005, but it was surely an improvement. However a feature was still missing. You couldn’t go offline when you wanted. You could only go offline when the solution was opened. So if you disconnected from the network and wanted to work offline you had to close and reopen the solution (which can take some time on large solutions).
TFS 2010 didn’t provide any love to the offline support and things are the same as in 2008.
If you miss the ability to go offline without closing/reopening the solution you can go to Visual Studio Gallery and install GoOffline Extension
The action is available at Files->Source Control to go online again, just use the use go online button available on Visual Studio