How do you use your Ipod as a PDA? How do you enter the information?
Well the iPod supports a few standard formats.
It supports TXT (Text files) for Notes.
It supports VCF for Contacts.
And it supports iCal for Calendar.
And iTunes itself has built in syncing with Outlook (not Outlook Express, just Outlook). If you're on a Mac it's even better as it can sync with iLife components.
There are extensions for both Thunderbird and Sunbird that allow you to sync contacts and caledar to your iPod.
Google Calendar also supports iCal exports of both public and private calendars.
It's not the easiest set of things to do, and there is no on-the-fly entry of data (like you can't make notes with your iPod, you can only read them). But it works. For example, my wife is on a few different prescriptions, and I keep a note about those handy on my iPod. This is useful for Hospital/Doctor visits.
I also keep my contacts in there as well, but I've only got some custom software to take my Yahoo Addressbook and convert it to VCF. It's not for public consumption, at least not yet.
I do have something to download google calendars, that is soon to be released (I have one more change I want to make), but that's about how I do it right now.