I use Verizon Wireless, and I "teather" my phone to my laptop if I'm using it anywhere I can't get an 802.11 connection. I currently use the LG VX6100 phone, and a couple months ago, I bought the Verizon Wireless "mobile office kit," which includes a USB cable, "VZAccess Manager" software, USB Modem drivers for the VX6100, "Venturi" image compression software (optional to use, doesn't make a huge difference in speed anyway), and a tool for backing up your phone's phonebook. The cost was $40 for the kit. There are cheaper options, but I went with the official option because I didn't have time for any hassles making it work. For my $40, I'm satisfied, because once the USB modem driver was properly installed, the connection worked on the first try (the driver took two or three tries because the installation software and XP were fighting each other--to make it work, I think I let XP handle the driver installation by manually pointing to the driver CD).
Verizon uses its own ISP. You're "supposed" to get a data plan, but you don't really need one if you only connect occasionally; every minute online just comes out of your voice minutes, a non-issue during nights and weekends (9:01 PM to 5:59 AM, all Saturday and Sunday, and several major national holidays). The important thing to understand is that it's kind of a "don't ask, don't tell" thing; if a rep figures out that you want to connect your laptop to the internet using your cell phone, they're "supposed" to sell you a data plan. But because it is allowed for occasional use, most of them don't care.
Verizon has three different types/speeds of data connection: QNC (quick network connection?) @ 14.4 Kbps, 1xRTT "National Access" @ "up to" 144 Kbps (overall speed is only slightly faster than 56K modem most of the time), and EV-DO "Broadband Access" @ "average speeds of 400 ? 700 kbps."
Currently, you've got to pay quite a bit for the Broadband Access, and the system isn't live in all areas yet. (Also, only a couple of their phones support EV-DO at the moment.) I understand, however, that where it is live, it totally rocks.
The two services that are "free" are QNC and NA. NA (the faster of the two) should work anywhere there is a native Verizon Wireless signal; QNC should work if you're in one of the vast non-Verizon "extended network" areas, which is technically roaming, except that there are no roaming charges.
The experience using NA is similar to, maybe a little faster than, using a 56K modem, but with a few seconds of extra latency if the connection has gone "dormant," meaning it wasn't active in the last 30 seconds or so. (Speed testing the NA connection shows anywhere from about 50 to 90Kbps, both up and down, with a strong cellular signal.) The QNC connection, as far as I can tell, doesn't go "dormant," but it's super slow otherwise. (Speed testing the QNC connection, as expected, showed exactly 14.4 Kbps.)
You can also dial up to your regular ISP using one of their regular phone numbers, but the connection is only going to be 14.4k. The only reason I can see to do this is for email only, if your ISP doesn't allow outside connections to their e-mail servers (a common tactic to prevent spammers from hijacking your email account remotely to send spam).
Conclusion: If your need for speed is relatively modest (if 50-90 kbps will do for occasional use), if you don't need access often during peak times, and if you don't want to pay extra per month, minute, or kilobyte, get Verizon Wireless. (Yes, it uses your peak minutes during peak times, but otherwise, it's essentially a free ISP on nights and weekends.)