Tethering the iPhone is going to be unbearably slow and require a fair amount of hacking.
My company has experimented with several of the USB cards and services from Verizon, Sprint, and AT&T. In Florida, we have found Verizon to be the fastest and most reliable. Typically these plans cost around $50 a month.
If you're not sure how you will like the service, I believe most of them offer a 14 day trial period. You can sign up and return the USB card and cancel the service within 14 days no questions asked.
You could also consider public WiFi. It's to be found all over the place in most cities. In rural areas, you're out of luck, but any wireless service can be iffy in the country. Public libraries, Panera Bread bakeries, most new McDonald's all have free WiFi. Starbucks has a pay service that isn't super cheap, but if you're in a major city in the US, there's one closeby, no matter where you are.