Mono 2.6.1 Release Notes
I would say that the main thing in this release is LINQ to SQL support, but more complex queries will not work, so the main area that is still not fully compatible is that. There are many more ways to solve (or a workaround) for this problem, either by simple ADO.NET (MySQL, Postgre SQL and SQL Server working fine), or NHibernate (not tested it, but I saw somewhere that it works). As far as I know, the Entity Framework is in Mono's plans.
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