Advance for Nurse Practitioners has a story on cell phone interference. The bottom line: cell phones are pretty okay, just keep them 3 feet away from a medical device.

Actually, it's not really that simple. The story goes on to recount the following horrors:

You don't need to imagine. Read "Don't Answer That Cell Phone!" in the June 2002 edition of the journal Nursing.
That article, penned by FDA officials, recounts how an ICU patient was
receiving epinephrine through an infusion pump when a visitor nearby
answered her cell phone. The pump suddenly increased the rate of its
drip, giving the patient "an unintended bolus of medication that led to
epinephrine toxicity."

FDA officials have also received reports of interference affecting
powered wheel chairs, monitoring equipment, hemodialysis devices and
cardiac devices.

As recently as 2004, a study concluded that cell phones "placed in
close proximity to some commercially available intensive care
ventilators can cause malfunctions, including irrecoverable cessation
of ventilation." Robert Kacmarek and colleagues found that
EMI is most likely to occur if a cellular phone is less than 30 cm from
a mechanical ventilator and is ringing.

The article goes on to describe all the new wireless technologies that are invading hospitals. The real bottom line is that there are many sources for electromagnetic interference. What's critical is that hospital staff is trained to recognize possible interference and know what to do in response.

Cell phones are here to stay, the genie's out of the bottle. Many hospitals have enacted policies that cell phones can be used anywhere as long as they stay 3 feet from a critical medical device. I've heard of many hospitals "holding the line" on cell phones, and some losening restrictions, but I've yet to hear about a hospital giving up more liberal policies for those that partially or completely ban cell phones.

UPDATE: Here's another cell-phone-in-hospitals story from the UK that says, "Hospital cell phone ban unnecessary"  - previous story here.