until the Linux distributions get some sense and stop fighting with each other and unite into a single distribution for a single modern target architecture (like 64-bit), and make it easy and fun to develop and distribute for Linux.
Well lets face it theres only really two, redhat and debian. Almost everyone using linux falls under these two umbrellas with a simple to use package system on both sides .rpm and .deb and the binaries will work on both as long as the dependencies are met.I would go out on a limb and say that is exactly what Ubuntu is doing, bringing Linux into a more useable mainstream standard that can be used by anyone (with the right hardware:). As for Redhat, thats technically a completely different operating system, a commercial one but with a more careful approach, but a similar vision.
That makes:
-OSX,
-Debian(& compatible)
-Redhat(& compatible)
-BSD
-Windows
..as the main OS platforms. People using others are probably know how to get stuff working "their" way (as they always have). So in reality there is only two Linux based operating systems to worry about development wise.