Many distributions include Mono. Each one has different requirements for the Mono packages, and often includes different patches.
Fedora
The source for Fedora packages are in
http://cvs.fedora.redhat.com/viewcvs/devel/mono/
There is also a branch of this for each version of Fedora. For example, fc5 package sources are at:
http://cvs.fedora.redhat.com/viewcvs/rpms/mono/FC-5/
Debian / Ubuntu
Debian and Ubuntu share the Mono package source from:
http://svn.debian.org/wsvn/pkg-mono/mono/trunk/debian/
Debian
Binary packages for Debian are available for Debian/Unstable and Debian/Testing:
http://packages.debian.org/unstable/interpreters/mono
Backports for Debian/Stable (Sarge):
http://www.backports.org/package.php?search=mono-jit
Ubuntu
Binary packages for Ubuntu live at: