Can anybody give more information  or references on the possible
interference of the use of cellular phones on monitoring or other
medical equipment?.
Do any guidelines or regulations exist in U.S. or U.K. ?

Thank you for your courtesy.

I know a little bit about the situation in the UK.  The UK Dept.
of Health produced a "Safety Action Bulletin" SAB(94)49 dated
November 1994 on this subject. Excerpt follows.  I've cross-posted
this to uk.telecom, where you might get further advice.

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         PORTABLE, CORDLESS AND CELLULAR TELEPHONES:
               INTERFERENCE WITH MEDICAL DEVICES

Summary
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The Department has received reports of mobile and cellular telephones
interfering with the operation of medical devices. Portable, cordless
and cellular telephones should not be used close to patient monitoring,
infusion, or life support equipment because such interference may effect
their normal operation with potentially serious patient consequences.
Whenever possible fixed communications should be used.

Actions
-------

[...]

Staff who are issued with portable, cordless or cellular telephones
(or two-way radios) should not use them close to patient monitoring,
infusion or life support equipment.

Patients, contractors and other visitors should be discouraged from using
such telephones in hospitals.

Managers should identify those areas (eg: ITU) where susceptible devices
may exist and areas adjacent to them (eg: reception) and ensure that
staff and visitors to those areas are aware of any restrictions in the
use of these telephones. for example consideration should be given to
the display of warning notices.

Background
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1.  Incidents involving interference have been reported.  Amongst these
were the reprogramming of an infusion pump resulting from a patient
using a mobile telephone, and the alteration of the tidal volume and
frequency setting of a ventilator caused by a contractor's mobile telephone.

[...]

3. [...] Because of the unpredictability of the effects...the Department
can give no advice on recommended safe distances [...]

4.  The Department is also concerned that any bans or restrictions
on the use of portable telephones should take account of the benefits
that the use of such systems may offer.  To allow these benefits to be
realised, use should be allowed wherever safety considerations permit.
Total bans effecting whole sites are not favoured because they are very
difficult to enforce and may quickly fall into disrepute (if mobile
phones are illegally used but no harmful effects are observed).  It is
thought that bans in limited but specific areas are more likely to
be effective.