I am pleased to announce that Mono C# compiler (gmcs) has now full C# 3.0 support. Most of the features has been available since Mono 1.2.6 release. However, with the upcoming Mono 2.0 release we will also support complex LINQ expressions and mainly expression trees which is fairly overlooked new feature with a lot of potential.
For anyone interested in compiling and running this LukeH's slightly extreme LINQ example I have good news. It compiles on Mono and it runs as fast as on .NET.