Here is Scott Guthrie's announcement.
The service packs are scheduled to RTM this summer.
I think the following are useful features:
- Faster application startup times
- .NET Framework Client Profile: only includes the client side libraries for a smaller download.
- ClickOnce improvements
- WPF Improvements
- The ADO.NET Entity Framework and LINQ to Entities (yay!)
- Astoria: a REST-based data service
- WCF improvements