Well, it wasn't too difficult to figure out, getting .Net apps to run on my G4 Mac.
Here are the steps I used:
- Install the mono framework: http://www.mono-project.com/Downloads
- Install the J2SE 5.0 runtime environment that the IDE I chose required: http://docs.info.apple.com/article.html?artnum=301073
- Then I installed the X-develop IDE: http://www.omnicore.com/register.jsp
I compiled and ran my "hello world" console app with no fuss whatsoever.