Visual Studio Team System will be the enterprise project support system from Microsoft. Unfortunately, Microsoft does not support client tools other than their own Visual Studio IDE on Windows. In an enterprise organisation it is common to use multiple technologies, and because of that, use different development tools as well.
Teamprise is a new product from the SourceGear company that is a 100% Java client for Team System. It can be used as an Eclipse plug-in or as a stand alone Eclipse based tool. Because it is Java/Eclipse it will run on Windows, Linux and the Mac OS X. It will be interesting if and how some advanced Team System features will work on another client platform. Teamprise could make it more appealing for large organisations to adopt Team System. Just be ready to pay another 799$ per user (current estimated price for Teamprise) on top of your Team System costs.