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I carry around two phones. Currently an iPhone 4 and iPhone 3GS. I just replaced a dumb phone with a used 3GS about a month ago, and I have not been put on a data plan. I fully expected to, and I guess it still could happen in the future. I wouldn't mind having one, but I am just as happy saving the money since I already carry around an iPhone 4.
Not sure how exactly I've avoided a data plan. I believe there may be a data block, but I don't think so because MMS works. I just mention that because the data on my dumb phone mysteriously stopped working a while back.
Also I don't have voicemail on that line. I'm not sure if having disabled voicemail somehow interferes with how they automatically detect the iPhone.
Also I had ordered the iPhone 4 for the line that currently doesn't have data. Before I activated it I called them I asked them to do an upgrade swap with my 3rd line and they did some messing around with the IMEI numbers in my account. Not sure if something got mixed up there either.
 
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Ok, then how about "like a sports car on regular unleaded"?

Better?
:)

Bad analogy. Lower octane fuel in a performance engine will increase cylinder temps at wide open throttle to the point where it would cause detonation and possibly engine failure.

The consequences are much more severe to the car than to the phone.
 
I have a 3G iPhone, jailbroken & unlocked on a prepaid (aka Go Phone) plan. I called AT&T and had them block data just in case I accidentally hit a button that requires data.

It can be done.
 
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