The ADO.NET team blog have posted about a number of vendors who are currently working on Linq-to-sql providers for other databases, some of these include Oracle, Informix, Ingres, Sybase, MySQL, PostgreSQL, DB2, Progress and Microsoft SQL Server databases.
I won't be getting too excited as most of these more due to come within 3 months of the official VS2008 release. Come to think of it, I think this is when most vendors are planning to release new versions, including other cool plug-ins like Resharper.