I have written a few articles at www.sqlservercentral.com on the new features and enhancements added to SSMS in SQL Server 2008. A couple of them are already published and you can find them at the URLs given below.
This is a very short article which shows the new splash screen of SQL Server 2008 and the new Activity Monitor. It also shows the enhancements to Query Plan window which helps generate scripts to create the missing indexes.
This explains the debugging support added to SSMS 2008. You can now debug a stored procedure, function or trigger just like you debug any other application using your favorite IDE.