In our company we are setting up TFS2010.  We have decided not to roll-out VS2010 by default.  Most of the developers still going to use VS2008.  Standard VS2008 can't connect to TFS2010.
But by installing the Visual Studio Team System 2008 Service Pack 1 Forward Compatibility Update for Team System 2010, they can connect.

Be sure that Vs2008 SP1 is installed.  Next download the Update here: http://www.microsoft.com/downloads/en/details.aspx?displaylang=en&FamilyID=cf13ea45-d17b-4edc-8e6c-6c5b208ec54d and install it.

After the installation, you can add your TFS2010 server in the Team Explorer 2008.

One remark:
Normally, when adding a server, you just type <Servername> and Team Explorer will add the port. To connect to TFS2010 to type:

http://<servername>:<port>/<web application path>/<Team Collection>.  See image for an example.



This will give you the list of all Team Projects under this collection in the way a developer is used to see it for TFS2008.  You can't leave the teamcollection out of it as VS2008 can't work with team collections.

Second remark:
Don't close the URL with a trailing backslash.  Team Explorer will fail making the connection and give you the message "Team Explorer is busy....".  This bug is introduced in the Team Explorer Forward Compatible Update.