if you have bluetooth and OSX leopard, and a blackberry, the Bluetooth modem tether script works excellent... even if the cell internet is slow by wifi or broadband standards.
I'm on T mobile's network (no anti modem rules, unlimited dataplan) using an unlocked cingular phone, with backdated Cingular software (newer software prevents the modem feature).
I previously had a Tmobile blackberry phone that was also on backdated Tmobile software for the same reason (back when i was on tiger), but someone thought they deserved it more than I...
I recently went across the southern US and other than a few dead areas, I was able to use my macbook's internet (slowly, but it works) whenever and whereever I needed it.
jsut be careful.
some providers get a bit peeved and charge big $ if they find you use your cell phone as a modem, so if you are Not on Tmobile try to get the card or adapter and a data account to be safe.
I have a treo 650 with Sprint. Using the phone as a modem is incredibly easy to setup and use. I think it's either 15 or 25 bux a month or so for their unlimited internet package. There is a phone as modem plan as well, but it runs about as much as a card. With the 700Ps and above DUN (dial up networking) is disabled unless your are paying for the PAM (phone as modem) plan. The 650, however, is not disabled. So if you have Sprint, pick up a treo 650 and car charger for cheap off eBay, and just throw it in your glove box. It connects through bluetooth. I have yet to have any complaints with Sprint, but I second what attack eagle was saying. I have heard of outrageous phone bills if they catch you. It's not fast, so you can't stream mp3's over the internet or anything, but for normal web surfing and email, it's sufficient. Although, I've heard with an iPhone you can setup routing rules, setup an ad-hoc network, and use the machine's wireless to get to the internet. With 3g it should be pretty decent speeds as well. That'd be the way to go. But I'd do a little research on that first. I'll be doing some fairly soon, so I'll post my findings in when I get to it.