If you used ASP.NET 2.0 SQL cache dependencies in early betas of the product with SQL Server 2005, you may be surprised when your code breaks on newer builds. You now have to call System.Data.SqlClient.SqlDependency.Start before using SQL cache dependencies. A good place to do it is in Application_Start.
There’s a helpful document on MSDN that describes the major changes between beta 2 and RTM. It’ll save you time when you tweak your beta 2 code to run on the actual Whidbey bits.