There will be a new official Mono build soon as part of the RC process. For now you should be able to clone the GitHub repo (http://github.com/markrendle/Simple.Data) and build the latest version.
Mono support is a requirement of the project, and will continue in all future versions.
Everything that works on Windows and MS.NET should work on Mono on OSX and Linux. If you find that something does not work, that is a bug and should be reported at http://github.com/markrendle/Simple.Data/Issues .
I have not personally got any projects using Simple.Data on Mono, so I can't offer a real testimony, but there are ~700 tests which all pass using Mono 2.10 on Mac OSX against a SQL Server DB.