When you install a new Microsoft operating system, chances are you are also installing the .NET Framework. This is great since it means your .NET applications should have the runtime files they need.
On the fedoraproject.org site, a legal issue has been closed regarding the inclusion of Mono with the Fedora project. On the 10th of January, this issue was closed with the note "Mono is in rawhide now." A link from this note provides the additional information that Mono is in rawhide and will be included in FC5, the Fedora Core.
Bottom line, you can now expect at least one non-Microsoft operating system to include a .NET runtime without any added effort of installing.