(disclaimer: I'm the author of this lib). LibNUI supports iPhone, Win32, Win64, MacOSX and Linux. It offers complete OS abstraction for Threads, Windowing, Unicode Strings, OpenGL / GLES / Direct3D GUIs and many widgets. You can use it in any GPL application or buy a commercial license. Check it out: http://libnui.net/

Since the original question was about development I just made some test with an lib I'm working on. I compile the same source with the same build system (gnu make + gcc 3) on a PIII 600 Linux box and an iBook G3 700 machine. The iBook compiles in 6m45s when the Linux box compiles the lib in 1m55s. I also tried to compile the code on an external FireWire 7200rpm HD with exactly the same results just be sure the laptop wasn't lagging on the disk side. I did the same compilation on my PIV 1.6Ghz Dell Laptop with VC.NET and it took 2m30s to compile the lib on the internal disk (WinXP Pro). I think compilation times are quite revelent to using a box as a dev machine or not and in my case I think the choice is quite easy to make.