I have done the PDA route pretty well and concluded that while it can do the job as an emergency solution (and I still carry a HP4700 for this reason) the screen is too small.to be any good.
I would buy a lightweight laptop, or one of the higher-end tiny "EEE" laptops which have recently come out, with windoze (not unix), and bluetooth, and use a mobile phone (also with bluetooth, and GPRS/3G) as a bluetooth-connected data modem.
There are certainly laptops around with a built-in GSM/GPRS/3G modem and I have a nice one (Thinkpad x60s 1704) which I use for ground tasks (flight planning etc) but this has the disadvantage that it needs its own SIM card, and due to the sporadic usage you will go for a PAYG one, which in turn results in ripoff data tariffs, especially on Vodafone (I am being ripped off £10/MB when abroad) to which most such laptop modems are locked. I have found a prog which should unlock mine but haven't used it yet because I want to get shot of my prepaid credit first. It is better to use one's normal phone as the modem because the SIM card is then shared, and one is much more likely to have a contract SIM in one's phone, and one gets much better data tarrifs on contracts than on PAYG.
With a laptop, one can run the whole lot on it, which saves a lot of messing around.