I don't - and are your PDAs going to have external keyboards too, or are you expecting the kids to write essays by stylus hunt'n'peck... ?
As to the sort of access point required, you can do RADIUS, policy-based authentication, that sort of thing on pretty economical Netgear APs, like the WG102, which also gives pretty good coverage. Can't do that via Bluetooth. Just had a look at a BT AP, by the way - 7 simultaneous users at a whole 723kbps!
Podcasts are already used in higher education settings, and there's no reason why they couldn't be used in schools.
As far as the sort of data being input, just consider tests; you can handle multiple choice and some-of-many answers pretty easily, and well-designed software can be adaptive to the responses received.