Despite the fact that the two have different types of training, in practical situations, nurse practitioners and physician assistants are increasingly being called upon to do similar work (particularly when the physician assistant has a master's degree). In many practices, the two are both conducting physical examinations, diagnosing illness and prescribing drugs. And in truth, the two take many of the same courses, though nurse practitioner training is focused on nursing, and PAs on the medical model. All of this is convenient for busy practices who need as much flexibility as possible in deploying their staff.
That being said, each have their own limitations. At least in Pennsylvania, nurse practitioners are supervised by doctors and available to respond to questions. However, PAs work directly for doctors and provide a limited scope of specified services. That's because their training differs meaningfully. To become an NP, one must have a bachelor's degree in nursing, then go on to get a master's in a subspecialty area. They also must get nationally-certified and state-licensed in their specialties, plus undergo periodic recertification--which includes the needs for continuing education credits. PAs, for their part, may not end up with a bachelor's degree, much less a masters, though such programs are available.
To learn more about this trend:
 - read this Pittsburgh Post-Gazette piece [1]
Related Articles:
PA law will expand nurse practitioner role. Report [1]
Links:
[1] http://www.post-gazette.com/pg/08072/864248-114.stm