Boy, was I wrong. I've been using the 3GS so much that by the end of the day, I've not only burned through the iPhone's battery, but I often burn through the external battery in my Mophie Juice Pack Air case as well. (I highly recommend the Mophie, by the way, which removes a subtle disincentive to using the device because you're worried you may run out of battery life.)
The following is a multi-part description of my own use of the iPhone 3GS. In my experience, the best way to learn to use a new gadget is to look over the shoulders of others, watch how they use it, ask lots of questions, and experiment constantly. (Most iPhone owners are familiar with the phenomenon of having someone grab their phone, scroll through the applications they have installed and ask, "What's that one do?")
So if you have an iPhone, and are a health care professional or patient, this will hopefully be helpful. Even if you don't have an iPhone, some of the applications I describe are available for other PDAs — or they soon will be, since the iPhone has significantly raised the bar for what a mobile device can accomplish.
(Original drafts of these posts were written on the iPhone app BlogPress.)