Lucky you, not getting charged for downloads and having free tethering!
How people get charged for data in the US varies widely. It depends on the carrier, and even when and how they signed up.
Right now if you're on Verizon getting an iPhone 4, you can get unlimited data for a flat price ($30). They allude to this being for a limited time only, however, and there may be plans in the future to change to capped data plans, but there's no specific timeframe for this yet. One would assume though that if you're on an unlimited plan now, you should be able to keep it.
Unlimited, for all intents and purposes, is unlimited: the flat fee pays for all of your usage on the iPhone, except tethering.
A personal hotspot (tethering) option on Verizon adds an additional $20 per month, but the fine print appears to limit you to 2GB of data per month via the tether. Not sure what happens if you exceed this amount.
On AT&T, it's a little more complex. If you were an iPhone user
before the iPhone 4 and upgraded, it was possible to "grandfather" the original iPhone 3G data plan, which was $30 for unlimited data. Same deal as Verizon: unlimited is unlimited use on the iPhone, except for tethering. Some people will (and have) erroneously argue that there is actually a 5GB cap on this plan, but that's not true: the 5GB cap is for laptop data plans.
The no-tethering thing is key here: On AT&T's unlimited plan, tethering is NOT allowed. AT&T has apparently been doing checks recently, and if you have jailbroken your iPhone and tether with it, AT&T will take notice and take you off this plan.
The unlimited plan is officially no longer available on AT&T, but there are loopholes and ways to get on it, if you know what to do/who to talk to.
The
new AT&T data plans (starting with the iPhone 4, if you were new and couldn't keep your old plan) are as follows: 200MB of data for $15 per month, or 2GB of data for $25 per month. You can ("legally") tether for an additional $20 per month, and this adds another 2GB allowance to your plan per month, bringing the total to 4GB.
On AT&T, data overage is $10 per additional GB.
In all of these cases, both uploading and downloading of data counts against whatever data allowance you have.
Of course, any data usage you do on your iPhone via WiFi does not count... including any data an AT&T iPhone user would use at AT&T WiFi hotspots.
Clear as mud?
