I also tried to investigate this last spring before a 3 week trip. Had just bought a new laptop, upgraded my fone to one that had internet access, purchased that precious $60 USB cable to "tether", and then spend quite a bit of time on the fone trying to work it out..... AT&T also did not let you use "minutes", instead everything was data transfer in KB... which as advised, just pulling up a website could be quite large, (example, look at this one-- click on File, then Properties, and you'll see)... then of course any clicking browsing, unwanted popups, well, I can see where that large bill came from.....--- I didn't even bother trying on the road... just used the fone to access Yahoo email once or twice... since direct thru the fone, was not too many KB, but wouldn't advise on a regular basis....
I've now switched over to Cingular(they've taken over AT&T), and like you, have given up on internet thru cell fone, until I hear they have a "minutes" plan.... for now, I doubt it.