Flight Safety,
I actually asked this question about the interface between simulator control and air data system at a lecture held by a sim engineer recently. IIRC they indeed do not use pressurised air to simulate atmospheric conditions but rather install modified altimeters etc which are controlled by servo motors. Both our memories agree so we're probably right.
I found it interesting that many simulators seem to be registered as actual aircraft, mainly to simplify the paperwork regarding the instrumentation and avionics. That way, they can take an instrument right out of the simulator and put it in an actual aircraft, should the need arise.
Cheers,
/Fred