Do you use the phone to connect the PDA to the internet?
Absolutely. The magic for this to work is Bluetooth, which allows you to connect the PDA to the phone wirelessly. It can be a fiddle to get it set up properly (it's getting better in each new generation of device) but once it has been done it works very well.
I can't advice on calling plans and networks in the US. I understand there are some which offer unlimited data access (subject to fair-use policies) alongside a voice service. T-Mobile do this is the US I believe, but I've no idea if their price or coverage is the best.