You should look at T-Mobile's prepaid versus TracFone. For very light use it costs under $10 per month. Get a $100 prepaid card which gives 1 year of service and 1000 minutes @ 10 cents per minute. Actually it is <10 cents per minute because by spending $100 they give you a 15% bonus. Texting costs 5 cents to receive and 10 cents to send, not as cheap as I'd like, and now having a full keyboard I find myself texting a lot more. I have no data plan, I use free wifi only. For navigation, caching google maps didn't work as well as I'd hoped (many levels of zoom, maybe theres a better caching app) but there are navigation apps that store the whole US/Canada locally on the phone. To get a T-Mobile SIM card I bought a T-Mobile Nokia 1661 for $18 at Target, which also comes with $30 of airtime. I stuck the SIM in my newly aquired iPhone 3GS after jailbreaking and unlocking it. For those of you wondering why such a cheapskate has a 3GS.... My iPhone 3GS itself was a lucky find for the cost of an iPod, checking craigslists in small towns can pay off ;)
TracFone, and their sister company Net10, have horrible customer service. If you can even get through. I couldn't, after trying several times and holding for up to 2.5 hours before giving up. A couple years ago these companies appeared to be the best deal to me. Rates are somewhat comparable with T-Mobile prepaid with pros and cons: there is a little higher monthly minimum, texting is a little cheaper (especially with Tracfone), and there are no roaming charges anywhere in the US to worry about with Net10. But you'll have to use the crappy phone that comes with the plan, even if it is a GSM phone using AT&T in your area, the SIM is somehow locked to that specific phone. They lock the USB so it can only be used for charging, you can't even upload your address book. (Guess what I was too lazy to write down before I lost my old phone?) For TracFone/net10 phones with a crappy web browser, they are locked to only work with their site, at an expensive rate. Just opening the browser costs you money...and with some of their phones having an internet hotkey you will probably find yourself opening it accidentally.
I may be interested in alternative GSM carriers in the future, especially those offering an inexpensive prepaid data plan. GSM carriers are few in the US, only AT&T, T-Mobile, and a small handful of companies who resell their services. AFAICT T-Mobile's 3G service will never work on the iPhone because it is on a different frequency, limiting iPhones to EDGE speeds with T-Mobile. I originally considered that popular T-Mobile "sidekick" unlimited data/txt plan until T-Mobile closed ports 80/443 last month (just when I got the iPhone). Creating a VPN as a workaround didn't sound like a bad idea, but read there may be complications switching back and forth to wifi, it started sounding like a PITA, and I didn't really want to spend the extra $ now for data anyways.
Next I need to apply for a Google Voice account, pay the few bucks to get a more memorable master phone #, and start using VOIP when on WIFI.
