So may be now you have finally got your hands on Beta 2 of TFS 2010 (for example, using one of the excellent VHDs prepared by Brian Randell) and it is time to see how your custom applications written for 2008 fare (do not worry, you will be fine!). Here are few notes that should make your life easier.
1. Starting with Visual Studio 2010, SDK does not include samples and documentation. TFS SDK is located on Code Gallery, separately from VS SDK documentation.
2. The backward compatibility of the APIs in 2010 is very good; so if you had an application written against TFS 2008 OM the chances are it will compile and work. You will still have to change references to assemblies to 2010 versions. Additional things to keep in mind
3. New functionality available in 2010 is mostly related to the new features introduced, viz.
So be adventurous and take Beta 2 for a ride! Oh, and most probably for a while you will have heterogeneous environment with 2008, 2010 or may be even 2005 clients etc. This post from WIT team blog will help in figuring out what will and will not work: TFS Server 2010 Beta 2 vs. 2005/2008 TFS client
And should you be in a tight corner (since currently existing documentation is somewhat meager) you might want to head to TFS 2010 Beta 2 MSDN forum.
Update: There is a good summary on Version Control API changes (that I forgot to mention) at Michal Malecki blog: part 1 and part 2