The journal for the International Anesthesia Research Society, Anesthesia & Analgesia, published a study
(full text $15) done in Stockholm on electromagnetic interference (EMI)
on medical devices. They tested "new" communications technologies like
cell phones (GPRS and UMTS) and wireless LANs (WLANs). The bottom line:

Devices using these technologies can be used safely in critical care
areas and during operations,
but direct contact between medical devices and wireless communication
devices ought to be avoided. In the case of GPRS, at a distance of 50
cm, it caused an older infusion pump to alarm and stop infusing; the
pump had to be reset. Also, 10 cases of interference with device
displays occurred. GPRS can be used safely at a distance of 1 m.
Terminals/cellular phones using these technologies should be allowed
without restriction in public areas because the risk of interference is

The Mayo Clinic recently announced their own study with a more conservative finding:

In their most recent analysis of cellular telephones and medical
equipment, Mayo Clinic researchers report in the October issue of Mayo
Clinic Proceedings that the cellular telephones tested did not
interfere with medical devices that were more than three feet away,
marking an improvement. In the current study, 44 percent of the devices
recorded some interference from the cellular telephones but the vast
majority of this interference should not have had any significance for
the patient.

You can listen to an audio news story (a pod-cast for the cognoscenti) on this topic at The Healthcare IT Guy's site, here.