I would suspect it is more of the latter right now, but as apps come out for the iPad, I think it's definitely going to be an asset for doctors, especially in the inpatient setting where doctors need their technology to travel with them.
I don't see iPads being very successful in an outpatient or ambulatory setting, mainly because most exam rooms are already equipped with desktops, and limited functionality may be more of a hindrance.
That's a vague statement, are they using them at work, or do doctors simply have a propensity for neat personal technology?