I know that Unity 3 is now running Mono 2.6, and that's wonderful -- it's such a huge performance and functionality increase from the older versions of Unity. However, the garbage collector still leaves something to be desired -- it's slower than that of .NET, and it's tending to run a bit more frequently than it should in my tests with our porting of AI War (from .NET 2.0), leading to periodic lag that shouldn't be there if it was working properly.
Mono 2.8 is the latest and greatest bleeding-edge version of Mono, and just check out those specs on the new garbage collector that they optionally include! This particular game of mine is particularly performance-intensive, and only targets the Mac and PC, so... is there a way for me to make Unity generate Standalone builds (and run in the unity editor) with Mono 2.8? For my other projects, probably it's preferred to stay on 2.6, but it would be really nice to be able to swap in Mono versions at will, with command line options for things such as using the incredible new garbage collector, etc.
Having a garbage collector that is so much lower-load and faster could only be a good thing for all Unity games, though.