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Most (all? *) apps for jailbroken devices are written in Objective-C / C / C++, just like the standard App Store apps. You can use an open-source tool-chain, but I recommend using XCode anyway. Check out Alex Whittemore's blog for all kinds of interesting info about developing for the jailbroken iPhone.


* I believe saurik ported Java to the iPhone. Check that, though.
 
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