In some cases we've been building tools that are specific to Linux for the desktop, and they only work on Linux, but I see two major projects that are wildly, wildly successful: Mozilla and OpenOffice, and those two programs are cross platform.
After releasing Mono 1.0, we started work on a new edition of Mono that will be released later in the year.
We're doing Mono because we care about upgrading the development platform, we care about language independence; and it's very nice two work on.