I asked about this a while back and it looks like there is no one doing this. I think it would be in Microsoft's best interests to provide TF servers to allow folks to easily take VSTS clients for a test drive.
This question cames up at the Microsoft Ascend 2005 training last month in Redmond and the answer that the instructor (who doesn't work for MSFT) had was basically "I asked them about that and they hadn't thought of that".