Entity Framework has been integrated into the Mono distribution as of 2.11.3 which means it can now be used even outside of the Windows platform. Mono also gets Async support on par with .NET 4.5.
In a somewhat brief announcement, Miguel said -
This is our fourth preview release of Mono 2.11. This version includes Microsoft's recently open sourced EntityFramework and has been updated to match the latest .NET 4.5 async support.
This should make it easier to port existing .NET projects dependent on EF to work on Mono. Mono already works with other open source ORMs such as NHibernate and Castle ActiveRecord. The other change announced, async support, was already in the 2.12 roadmap – although expected, this is an important change since async is needed before some features such as ASP.NET MVC 4 can be ported to Mono.
Note that this is still an alpha/preview release which is potentially unstable, and the features will be available for production use when 2.12 is released.