I'm still having problems with mod_mono, and apache. For no apparent reason, mod_mono stops communicating with the mod-mono-server process causing "Service Unavailable" errors to be returned by apache.
In another classic grope-in-the-dark move I've decided to build mod_mono from the daily builds on the anonymous mono SVN server.
I'm not sure that this will resolve the error at all, but for future reference I'm outlining the steps below (official instructions here: http://www.mono-project.com/AnonSVN):
Firstly, you'll need SVN (SubVersion) installed on your linux box. You can do this on Ubuntu with the following command:
apt-get install subversion
The next step is retrieving the sources from the anonymous Mono subversion repository.
svn co svn://anonsvn.mono-project.com/source/trunk/mod_mono
svn co svn://anonsvn.mono-project.com/source/trunk/xsp
You need to build a new version of XSP too, as mod_mono will fail to run if the XSP and mod_mono versions differ.
Once the sources have downloaded, change into the "xsp" folder and run:
./autogen.sh --prefix=/usr/local; make; make install;
Do the same for the mod_mono folder.
And you should be done.