For Linux, there is a prebuilt GCC cross-compiler (from publicly available Apple’s modified GCC sources).
https://launchpad.net/~flosoft/+archive/cross-apple
Update for 2015
- After so many years, the industry-standard IDE now supports OSX/iOS/Android.
http://channel9.msdn.com/Events/Visual-Studio/Connect-event-2014/311
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Embarcadero’s RadStudio also supports building OSX/iOS/Android apps on Windows.
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This answer by Thomas also provides a cross-compilation tool.
For all these options you still need a real mac/i-device to test the application.