I think that the IPAQ was maybe designed prior to the wide availability of those tiny PC-card hard disks. I expect that new upcoming PDAs will start incorporating those disks soon.
I doubt it for a few reasons:
1. Cost. Those drives are a good chunk of the iPods price, everything else is pretty cheep. PDAs already have other expensive parts like the screen.
2. Battery life. The iPod probably does similar tricks like the empeg to keep the hard drive spun down as much as possible, allowing it to hit the 10 hour mark for battery life. PDAs on the other hand would have much more random access to data, and would decrease battery life quite a bit.
3. Demand. For those who really need that much space on the go, expansion sleeves for the iPaq are available with a second battery. But for the majority of PDA users, gigabytes of data is not a necessity. (Though an increase to 256 and such might be nice).