In the RTM of Visual Studio 2008 and .NET 3.5, LINQ to SQL will support only Microsoft SQL Server and Microsoft SQL Server CE. There are no plans to support other databases. LINQ to Entities (the LINQ interface to ADO.NET Entity Framework) will support any database supported by ADO.NET Entity Framework. In this case there is a documentation available to build a ADO.NET Entities provider, thus each SQL vendor can implement its own ADO.NET Entities driver, allowing LINQ to Entities to operate with several differents RDBMS.
This came at a cost: LINQ to SQL should be faster than LINQ to Entities, but the product is still in beta and it's too early to do real benchmarks.
Finally, ADO.NET Entity Framework will be shipped some months later than Visual Studio 2008 and .NET Framework 3.5.
Marco