Mono Adds Mac OS X Package
Good news for those of you who've went through the pain of trying to get Mono installed on Mac OS X: the team has quietly added a Mac OS X package. If you previously installed to /usr/local, however, be aware that the packaged version installs to /opt/local and adjust any paths accordingly. The Beta-1 Windows installer has also been fixed; download it here.
It's in it's first beta. 1.0 is due at the end of June. Here's the story. And the roadmap
Therefore, the safe-but-annoying choice is to put your 3rd party stuff somewhere else. For example, Fink defaults to the (previously nonexistent) /sw directory. Likewise, /opt does not exist in OSX (unless you install this Mono package)