Your iPhone plan with AT&T can go over the 5GB limit and you will face additional charges. This is why tethering to an iPhone is a bad idea if AT&T wants to clamp down on data usage.
T-Mobile started offering UNLIMITED 3G data today but I am not clear how you can qualify for it.
My guess this move was to take on AT&T's iPad plan. I think TMOs's plan is a contract.
SPRINT offers UNLIMITED 4G data but their 4G modem can and does fall back to 3G. Sprint customers have gotten some horrific data bills when they thought they were good for unlimited 4G data. It is in the fine print with Sprint.
Sprint's plan is a two year contract with ETF.