Can you connect to TFS 2010 server using 2008 client? Sure you can! There are couple of gotchas to be aware of, specifically you’d be better off installing an update for TFS 2008; if you do not the following message comes up:
Grant Holliday wrote very detailed post on what needs to be installed and what to do if for some reason you cannot install the update.
However, I wanted to mention this in context of custom tools that use object model. While connecting to TFS 2010 with Team Explorer 2008 will produce the message above (and will require extra steps such as update installation to get TE working), the message won’t appear for your custom tools. In fact, if the connection to TFS 2010 is defined in registry you will connect and may not know the difference from inside of your tool.
Thus if you have custom tools using 2008 OM connecting to TFS 2010, it is your responsibility that the update gets installed correctly on machines with the tools.