In theory, connecting with a TFS 2010 server from Team Explorer 2008 should not be a problem. You only have to specify the full url (eg. http://servername:8080/tfs) instead of only the servername.
For most of our developers this worked fine, but for some of them this kept failing even after installing the Forward Compatibility Pack. Today I finally found the root cause of this issue. It was caused by the order in which components were installed on the developer’s machine.
They first installed Visual Studio 2008, then upgraded to SP1 and then installed Team Explorer 2008. The effect is that the service pack isn’t applied on the Team Explorer. After a rerun of the SP1 installation everything worked fine.