Hi,
I'm a big fan of Mono myself, it does work but I'm not ready to bet any mission critical ASP.NET apps on it as a platform yet. I'm optimistic about the up coming 2.0 release of Mono, and hope in the future to be able to recommend it without hesitation. If your app is not mission critical it might be a reasonable choice, I'm just saying I would not put my job in jeopardy by betting the farm on it.
Best,
Joe