Neumont ORM Architect (NORMA) for Visual Studio is a free, open source ORM tool now released as a "community technology preview". This release has a diagram editor that supports the new ORM2 notation to include mapping of the ORM schema to 4 RDBMSs (MS SQL Server 2005, DB2, Oracle, and PostgresSQL).
There are still some gaps in functionality (schema generation and verbalization currently only supports basic ORM constraints) but it seems to be a great start!
Note: NORMA is an add-on to Microsoft Visual Studio 2005, so you need to have that installed before you can use it.