Someone asked a question today about getting the URI for a Team Project. James Manning's Team Project code sample showed how to do it using the API. However, that was overkill in this case. Bill Essary pointed out that you can select the Team Project in Team Explorer, right-click on it, and choose Properties (rather than right-click, you could just hit F4). The Properties window shows the URI as the Url property (yes, apparently no one noticed that).
The URI you get will look something like the following. All Team Foundation URI's start with "vstfs:///" that will be tranlated to actual server URL's as needed.
The URI isn't needed for any normal day-to-day tasks. Occasionally, you may have the need for it for some advanced admin activity. TfsSecurity, for example, can accept Team Project URI's for some options.