Unfortunately the regulation changes are well behind the curve. The proposed rules which should come into force March 2010 IIRC should (will?) prevent current sims being used without significant upgrades. Visual systems will require a minimum of 200 x 40 FOV and “a fully integrated ATC environment”.
This ATC requirement is the challenge. The systems I’ve seen so far are nowhere near sufficient. Speech recognition and message broadcasting to cater an infinite number of (English) accents and regional ATC phraseology ‘peculiarities’ are probably the greatest technical challenges. However, already there are basic systems available that do indeed offer such an albeit limited ATC environment realistically integrated with visual airborne and ground traffic, TCAS traffic, ATIS, WX etc. These systems will evolve and I suspect within the next 5 years or so the simulation industry will develop systems that will fully comply with the requirements.
Until then, it is up to the various National Aviation Authorities to ensure current MPL programs reflect the present synthetic training limitations.