mono 188.8.131.52 is in debian sid (unstable dist) at the moment, and mono 2.4 is in debian squeeze (testing).
you could add lines to your sources.list for either testing or unstable distribution (or both), and set apt's priority rules to use only stable unless you explicitly ask for a package from testing or unstable (e.g. "apt-get -t unstable install mono-2.0-devel" - this will install mono-2.0-devel from unstable along with all versioned dependancies)
while it's possible - indeed, quite easy - to have a mostly-"stable" system with only a handful of packages from testing or unstable, IMO if you want to keep up with the bleeding-edge of software then you're better off just upgrading to debian unstable than sticking with stable, and getting into the habit of running 'apt-get dist-upgrade' semi-regularly. it'll be less hassle and there'll be less weird incompatibilities and other oddities (that's been my experience using debian for the last 15 years, anyway)
the same applies for ubuntu too, although with the 6-monthly ubuntu releases you don't need to wait as long to upgrade to the next supported release as you do in debian.