I cancelled my iPhone data plan 8 months later afteR I got my 3GS. I was under the impression you're tied with teh iPhone data plan forever. It's really just if you use the iPhone. I suppose 8 months is less weird than 3 days, but you can just say you're switching back to an old phone. You only need the data plan if you use the iPhone, and from what I read in the contract, the 2 year contract is for voice, and does not require you to have a data plan.
If you are doing it for $199, you can make a mass profit. I'm pretty confident even buying it at $599 you can make a hefty profit. I've seen iPhones go for $1000 the first few days.
If you're doing that and using the iPhone w/o a data plan, I would definitely use unlockit.co.nz or something to disable the APN. Too risky for accidental data. I was OK with my Symbian phone overseas as we're used to having to accept every new connection to data, but I'm wondering how I'll deal with my Android phone soon. I might have to just use my old S60 phone. Android pushes/pulls too much data without me knowing. LOL.