I don't believe there are PAYG internet-only options.
I have been using my treo and it's data plan for internet in my car, it works so so, but between the less than perfect performance and the cost i might be getting rid of the data service. Does anyone know if a pay as you go PC card exists? I really can't justify $40/month for the occasional weather report or traffic notification.
in australia there are heaps if you can get ahold of a usb modem and a PAYG sim as long as the apn is correct it should work fine
You used to be able to get a Cingular Boost phone, get a cheap talk plan (and not use it), then get unlimited data plan added to it for $19.99/mo, and configure it as a modem. Now, iirc, they charge $20 / 100 MB. Might be an option for a cheap connection if you anticipate low usage.
I have too much time and too little aggravation in my life, so I built a carPC. ;)