What have you got against guinea pigs? They are kinda cute after all.
At any rate, I cannot say that I have seen any major projects that have been cross platform compiled with .NET. But, then again, I have to admit that I have not really researched it. That said, I think that, perhaps, a web-based solution to the presentation layer may be the way to go. Or, more to the point, several different options for presentation would be good — don’t like java, use TK — don’t like Tk, use a browser. If the other layers in the hierarchy are coded correctly, the particulars of the presentation should not be a problem…
Also with reference to the guinea pig — it wouldn’t be the first time…LOL