There just isn't another camera that has the same connections to be an easy swap, then there's camera firmware, and getting the phone to speak to a camera not made for it.
It would be possible for the kind of guy who actually designs the electronics of these phones, not a job for you or I.
Basically, if you want a better camera then you have to buy a better phone. A shame I know because the i9000 is a great little phone.
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