There have been a number of studies recently that indicate that for the most part cell phones do not pose a danger around most medical equipment in hospitals (this study is from 2007 from the Mayo Clinic:
http://www.mayoclinic.org/news2007-rst/3958.html). But I believe there are some types of equipment that can experience interference, and hospitals find it easier to maintain an outright ban than try to finesse the whole thing.
Having spent entirely too much time in hospitals since the first of the year, I can say that no one ever tried to stop me from using my cell phone in my room, but I often saw signs in various clinics and testing facilities (CT/PET scan, X-ray, etc.) that said cell phones had to be turned off.