A cool (if incredibly geeky) thing I did with this to impress some friends:
Get a Tandy TRS-80 Model 100 or similar device with internal modem. Make sure it has the cord to actually attach the modem - if there's no cord, you're stuck here (explanation: the modem in the Tandy used a jack that's not RJ-11, you need a cord to convert. You can make one fairly easily if you can find the plug with solder terminals)
Shorten the cord considerably. We're talking "down to a few inches" here.
Build an acoustic coupler from a cheap landline phone using directions on the Web - but don't bother with the original phone body, stick it in a project box, the smaller the better. Make sure you have a good power source, and wire an on/off switch in there somewhere.
Here's the tricky part. Attach the coupler to your cell phone - I did it with some headphone cup things with the headphones taken out and a lot of duct tape. You can buy these separately.
Connect up the coupler to the Tandy and switch the bottom switches to "ACL" and "CAL" (Acoustic Coupler and Call (as opposed to Answer) respectively - the built in modem uses rotary-phone style clicks to dial, which won't work on a cell)
Tape the cell phone to the coupler if you haven't already, and tape the coupler to the back of the Tandy (the reason for the tiny enclosure)
Either find or set up a BBS or computer or something to connect to. I went the old-school way - I set (for a very short time) up a computer on what used to be the second line we used for dial-up and allowed computers to dial into it to get a terminal with all the usual Linux stuff, plus a simple Web browser (can't remember the one, it was very simple and fit fairly well on the tiny screen)
Head into TELCOM and do STAT M8N1E to set the modem to what you want. Dial the number on the phone and hit (I think) TERM to enter Terminal mode, which will make the Tandy listen to the line and check out any signals on it.
At this point you should have a Tandy with wireless Internet (or at least BBS, if you didn't set up a shell account on a dial-up computer) access. It's quite an interesting experience though not particularly useful - I actually bought one of those crappy prepaid phones to do this with for $6 on sale (I don't have or use a cell regularly)
One thing though - if you set up your own, set it to a very low baud rate - like 75 baud. The lower the rate, the more reliably you can connect, though it will be sloooow. Connecting at 300 baud worked, for some strange reason, for me, but was extremely buggy and ended up with garbage all over the terminal screen. Sentences like "%wset) you$Qfr ba!~K very lowwww." were common. At lower rates, the cell compression is less devastating to the modem - though I never got it to work perfectly.
All in all, a fun project if you have a Tandy 100 already - if you don't, getting one will probably be expensive and not worth it, though if you have money to burn they're nice machines to play around with. The nice thing about using the Tandy for this is it was designed with very low speeds in mind, and having a shell account on a computer with a decent Internet connection means that you can use the 'net at high speeds on a low speed computer, assuming you use it only for text.
SIDE NOTE:
A lot of this applies with any laptop, especially the bit with the shell accounts. If you're incredibly determined to use your cell phone in a stupid way to connect to the 'net, this is probably the fastest.