Do electrical heating floor systems throw off high levels of
electromagnetic radiation (EMF, measured in Milligauss)?

I have had this same question myself so I'm looking forward to the
answer!!  It would help if you defined "high levels."  What you are
asking for is the magnetic field as opposed to the electrical field
which may also be there.  If a person planned to sleep on the floor
then anything over 2mG would be "high" as that is a general standard
for a healthy environment (although some countries call that "high"). 
 If the person was walking across the heated floor a few times a day
with shoes on then "high" would take on a new definition.  Walking on
the sidewalks of New York will give you 50mG or more a lot of the
time.  I had to move from an apartment because the general gaussmeter
reading was 20 mG because I was sensitive to that level.  The utility
company told me the lots of people live in 100 mG spaces so they
thought that 100 mG was "low enough."

Earth?s static field strength is about 0.3 gauss and it seems to do us
no harm. EMF radiation from underfloor heating is extremely low at
50Hz or 60Hz so a few tens of meters of cable exhibit a radiation
resistance of megohms. Plus, the pattern of the cabling, to a large
extent, cancels the field.

Medical problems which arise from mains frequency radiation are almost
always caused by worrying about it, rather than the radiation itself
:-)

Best

There's no reason for electrical heating cables to give off any more
field strength than ordinary cables. The 'to' and 'from' (phase and
neutral) conductors run closely together thoughout the cable length,
so the two conductors cancel out each other. They are also usually
encased in earthy concrete screed.

If the wiring was run as single-core wiring in a loop round the room,
then there would be a considerable field generated - this is the
principle behind induction loops for hearing aid users.

Most people don't worry about ordinary electric cables or induction
loops in buildings.

Owain

The hazzard of 60 hertz electromagnetic is considered negligible by
nearly all main stream scientists and engineers. Because of some
alarmist books and articles on this subject, some efforts have been
made to reduce human exposure. Floor heating can be designed to
minimise exposure, but design changes are costly, so some new
instalations, expose people to ten or more times the levels of the
best designs. Even so, the level likely is not harmful even if you
regularly sleep naked on the bare floor. In the standing or walking
position the average body exposure is reduced by about 100 times from
the exposure of laying on the bare floor. This is because
electromagnetic radiation decreases as the square of the distance.
 The magnetic field of the Earth is a static magnetic field which is
not the same thing as a 60 hertz or higher frequency electomagnetic
field.
 Modern humans are typically exposed to much higher frequencies than
60 hertz electomagnetic radiaion, from hundreds of sources stronger
than floor heating. A few scientests and engineers are conscerned
about some of these frequencies, but most consider 60 hertz harmless. 
 Neil