The version of Visio that comes with VS Enterprise Architect has a forward-engineer feature that will generate SQL. There is also a type library for the modelling engine, but (on older versions at least) it won't extract certain items such as comments. However, the generated SQL has the comments in a fairly simple structure that does facilitate parsing the generated SQL.
You can get older versions of VS enterprise architect on E-bay for not very much money (I think mine cost about £250).
One caveat for reverse-engineers is that all pre-VS2005 visio DB modelling engines will not play nicely with the SQL Server 2005 native client. You need to either script out the database and re-load it on a SQL2000 server (dealing with SQL2005 specific features such as schemas is left as an exercise for the reader) or get a more recent version.
