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There's no reason..

..why it couldn't be done, as far as I know.

iPhone 3GS has ARM11 CPU, and Android has already been ported to that CPU.

iPhone 3GS has PowerVR SGX 535 GPU, and OS support for it includes Linux, Symbian, Android and Microsoft WinCE/Vista/XP, and (of course) OSX (Iphone). So no problem there either.

I bet the sound chip is something generic as well.

I'm pretty sure someone has an iPhone 3GS running Android as an operating system already :D
 
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