The data plan is not necessary whether your iPhone is unlocked or not, and unless you're completely against the idea of a three-year contract, I wouldn't even worry too much about cancelling.
You can get the iPhone 3G from Rogers for the subsidized price without a data plan as long as your voice plan is over $30, which in your case it is, and you're eligible for a hardware upgrade (ie, ahven't gotten a new phone in the last 12 months). Note that the $199/$299 pricing is the
best possible pricing, and many discovered that their pricing was not nearly as good as that for various reasons, so you may want to check into that before you make a decision.
They'll try to tell you that you should get a voice plan, but it's really not necessary to do so. In that case, they will make you sign a waiver acknowledging that you refused the data plan and will instead pay $0.05/kb ($50/MB
) for any data that you happen to use.
Further, I don't think that Rogers unlocks your iPhone just because you cancel your contract, and at this point there are no other unlocking solutions for the iPhone 3G... It has only been jailbroken so far and not unlocked.
The only benefit to cancelling your contract right away is that you would get out of the three-year obligation, but then again you can do that at any time
anyway.
If you
are going to opt-out of data, however, you should take steps to turn OFF data on your iPhone so you're not accidentally charged for it. Jailbreaking your iPhone will provide options to do this, or alternatively you can use Apple's own iPhone Configuration Utility to reset your data configuration (APN) to an invalid value so that the iPhone can't "log on" to the Rogers data network.