If you want to use them add this to your /etc/apt/sources.list: deb http://debian.meebey.net/etch-backports /
After that run: apt-get update
Now using, Synaptic Package Manager, search for Mono and you'll find Mono 1.9.1
Ubuntu 8.04 only comes with the 5 month old Mono 1.2.6 (yet includes the recent MonoDevelop 1.0) and it's a huge hassle compiling Mono from source (it really shouldn't be, but it is). These packages are intended for Debian Etch, but may work under Ubuntu 7.10 (Gutsy Gibbon) and Ubuntu 8.04 (Hardy Heron). This also includes Moonlight (Silverlight for Linux) related development packages. If you just need to run Moonlight, not develop for it, then check out the prepackaged Moonlight builds.
You can also use this to install MonoDevelop 1.0, but that's already in Ubuntu repositories (for 8.04).
I hope this is useful for some people developing Mono applications on Ubuntu Linux. If there are any issues, contact Mirco 'meebey' Bauer, the maintainer of those packages.
This is on Mono's page, Other Downloads, on their wiki.