Seems pretty esoteric. Seems to be for developers who actually care about other platforms.
.NET is good, but it will take a long time to really hit other platforms as code bases for specific things are developed. e.g. C# ziplib. Those sorts of things are extremely useful in programming. LGPL is great for Windows developers because they have a large potential user base. For the other side, it doesn't matter too much because their motivations are different or they are working in large projects that have significant commercial potential. Why do you think there are so many job ads for people who can program in C or Java on Unix platforms?
I'd like to try playing with Mono though, because it looks like you can reuse a lot of code across platforms. It's only in version 1.0 now, so likely it's good enough to use commercially if you test well across platforms. I don't think it's worth it if you just want to play into 1 basket of eggs though...
Overall, definitely something I'm willing to try.