I just noticed that the guys on the ADO .NET Entity Framework blog, has given links to some of the database providers (until now, there was just the SQL Server Provider).
Here's the link to the article: Click here

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From reading the post, it looks like there are new providers for:
- Oracle
- MySQL
- PostgreSQL
- SQLLite
- DB2
Keep in mind that these providers are built to run with Beta 3 of the ADO.NET Entity Framework. If you are reading this after May 2008, you'll want to check to see if there are updates available.
~ Robert Shelton