Just publicly announced, Visual Studio SP1 delivers various improvements to Visual Studio 2008 such as support SQL Server 2008 and new ADO.NET features such as the Entity Framework.
In addition, there are numerous improvements to the WCF Templates for Silverlight projects, along with a number of general debugging and Intellisense updates.
SP1 also includes fixes to improve the stability, performance and security of many areas of the product.
Note that this service pack will not address BizTalk Server 2006 designer support. IDE support for BTS 2006 will come in the RC3 release, previously announced here.
More info here: http://msdn.microsoft.com/en-us/vstudio/products/cc533447.aspx