Mono 2.8 was released on the 22nd August 2010 this is before ubuntu 10.10 was released however ubuntu 11.04 is shipping with mono 2.6 which is really very old now.
Why is this?
Novell who is the major sponsor of mono has chosen not to provide Ubuntu packages. The packages used in Ubuntu come from the community. The Community has not yet packaged ubuntu 2.8 and so ubuntu 11.04 is stuck using the old mono 2.6 packages.
Maybe this is a solution
The Debian Mono Group packages Mono for both Debian and Ubuntu. Our focus is on providing a quality and stable experience for users. This does not always mean having the latest version available at all times. Remember that Mono is also the basis for a large number of applications, including some which are in the default install. It's therefore critical that it functions as well as possible. Basically “new” isn't always what is best for the distribution.
Furthermore, packaging and QAing a major upstream revision is a huge effort, and as the team is comprised entirely of volunteers, this has to be done when time presents itself.
The groundwork towards making 2.10 available (rc1 was recently released) is under way.
I have ubuntu 11.04, with mono 2.6.7 - see http://mono-project.com/DistroPackages/Ubuntu
To get the latest ironpython 2.7.1 working, I have to update mono.
So I install mono 2.10.5, nicely build by Jo Shields on http://badgerports.org/
He said it is for ubuntu 10.04, but knowing from the first link that 10.04 and 11.04 both used mono 2.6.7, I try the update and bingo ! no errors, and ironpython starts :