I too have an ipaq., and the option like this would be awesome.
Just to brag (or at least it was when i got it) i have an hp IPaq 5550
now,
the options i see as said before, broadband wireless router then use pppoe for connecting. I think that might actually work, and i will try it because i just need the ethernet cable, see no reason why it should not since if my hub/router supports broadband dialup it should work with the wireless pppoe. Now one has to lug 3 things all over, and not just one

actually, i KNOW this will work because I have seen it work with adsl. Gorefest has it set up at his place like this, but it was a bit of a problem since i could use my ipaq from the street (before the wep encryption)
Ipaq that i have has a use interface but at the bottom is a bit proprietry connection for hte cradle.
Also, the Human interface stack is a bit proprietry to HP, they didnt use microsoft's one, which means that even if your device works with usb stuff will have to be tested. I will give more info, i read it somewhere, never needed to research it, i know it was an issue with bluetooth keyboards. I speak under correction because of lack of knowledge.
The thing i think might actually work better is getting something that will make your USB port to act like a com port, and that might work better since we know that you can use the modem like , well, a modem and IPAQ supports external modems. the connection might be not perfect but one could make one's own.http://www.ramelectronics.net/html/usb_serial.htm Drivers are the big no no, would have to write one's own for ipaq i think. I know of one person who could.
The other option I think we should explore is the PCMCIA jacket. This option might work the best. you can get a PCMCIA jacket for the ipaq, if i am not mistaken and this way you can possibly use a PCMCIA modem that would possible just be seen as another modem in your device. The problems here are drivers, once again i know to little of PCMCIA and the JACKETS to know of it will work.
http://www.expansys.com/product.asp?code=PCS-PDA-4054
http://www.expansys.com/product.asp?code=216198-B21
I bet you if we installed linux on the IPAQ, TheRodent would get it working within a week.
I didnt give any solution, just brainstorming possible unexplored ways.
btw, you guys know NASA is looking into making a Warp Drive , ala star trek style. brainstorm.