As a programmer, my thought is that anyone willing to invest time in learning an OS X dev language, should just invest that time in Obj-C.
-- Obj-C is the established standard, fully supported by Xcode.
-- There's a wide range of libraries and existing Obj-C code to leverage.
-- Writing in OS X's primary language, with Apple's official dev tools, to Apple coding guidelines greatly increases your odds of being compatible with OS 10.7 and future evolutions.
That said, Obj-C syntax is fairly awkward for coders used to Java, C++, etc...
If that's a hurdle, then consider developing in Adobe Flash/Flex Actionscript. Actionscript 3 is a clean, evolved language. Flash projects can be deployed as cross-platform (Mac/Win/Linux) desktop apps, web-based Flash, and this summer (via forthcoming CS5) also native iPhone binaries.
If you want to target more than just the Mac market, it's an easy win-win.