In order to access the new Team Foundation Server 2010 from your current Team Explorer 2008 you will need to install the Visual Studio 2008 Service Pack 1. Several customers were complaining that even with the service pack installed they couldn't manage to connect.
The problem is the order of the installation. For many customers, the orders was:
1) Visual Studio SP12) Team Explorer 2008
but if you have installed Team Explorer 2008 after the SP1 the changes are not in place. In order to sort out this just reinstall the SP1 and it will work.