Ubuntu is developed on a 6 month rolling release cycle so even if you’re running the latest alpha, 11.04, natty, you still only get mono 2.6.7. If you want to target .NET 4 or simply want to use the latest version of Mono, then you’ll need to compile mono from source. My colleague, firegrass, has written a lovely script which does all the heavy lifting for you, you can pull the latest version here
The script will ensure the correct dependencies are installed, check everything out from source, build in the correct order and importantly install all this in a parallel environment so that you still have access to the repo version of mono for applications like banshee, f-spot etc. You can even ask the script to pause after each install to review the progress. Aswell as the latest version of Mono you will get the Monodevelop trunk which includes support for git amongst other nice features.
If you clone the repo you can also keep up to date with bleeding edge changes, as the script is used inhouse almost daily to keep upto date with the latest from the Mono project. The script elegantly supports being rerun regularly to update your install. If a scenario is not covered pull requests and questions are more than welcome :)