On the ADO.NET Team blog there's a list of changes between SP1Beta & RTM: What's new in the VS 2008 SP1?
The designer got over 200 bug fixes including a bunch that help with source control and a few of the annoying validation error messages that are incorrect (and send you chasing down a problem that doesn't exist) have been fixed.
There are a bunch of fixes to the EntityDataSource. What I don't see on there is that you can now combine the Include (was IncludePath) and InheritedTypes. I complained about this when I first got my hands on the EDS so was happy to see those two working together. Events have changed a lot. You can see a list of breaking changes in the VS2008 readme file. Or just let Visual Studio point them all out to you, like I did.
There are also some changes in the basic Object Services model with improvements for folks doing databinding because the navigation properties are now available. Happy to see that IsLoaded is fixed so that it correctly reports its status. I've had to overlook the property because I didn't think I could count on it.
