Getting Started With Windows Client Programming
Getting Started With OS X Client Programming
Getting Started With Ubuntu Client Programming
How easy is it to get started with Ubuntu development?
The MS site has featured books, introductions, guided tours, videos, blogs, howtos, and presentations on programming for VS and .NET, making writing software for the platform as simple as possible.
The Apple site has overviews, guides, sample code, and tours for Obj-C and XCode, again making writing software simple.
The Ubuntu site has a description of the political process and details on packaging existing software. It does not, however talk at all about how to develop for new software for Ubuntu. They are pretty much just interested in bug fixes.
Ubuntu needs a developer community. Canonical is promoting that hard for 10.04. It's still too hard to learn to develop for Ubuntu.