I once heard of a guy managed to install original Microsoft .Net Framework on Linux and successfully run Mono-incompatible .Net applications using bleeding-edge MS C# features. Any idea how could he do it?
Actually, .NET 4 has been implemented in Mono. There just hasn't been a release, which will probably be very soon. Mono doesn't release beta's due to the fact that you can grab from source. If you would like to run .NET 4 now, then all you need to do is grab Mono from SVN and build it. The link that @Mike Cellini gave states it at the bottom. If you want to start developing or run 4.0 applications against Mono, then this will be your best bet at the moment. Unless you want to wait for a packaged release. There's plenty of documentation on Mono's site for building from source, which really isn't any different than building a release tarball.