I tried Beta 3 the other day. I wrote a console app in VB.NET 2003 on a Windows computer, then copied it across to my Mac and ran it via Mono. It worked 😀
A GUI app, on the other hand, complained that Winelib was missing. After several attempts, I couldn't get it working 🙁
I haven't any experience on this yet, even on Linux. But you might want to take a look at mod_mono and see if it compiles on Mac OS X (last time I tried I couldn't, but that was with Beta 1).