I am a (full scale) student helicopter pilot and most of the replies I'v read here are acurate. The only helicopters rated for any negative G's at all are the AH-64 apache and the new Commanche scheduled to be in service by 2006(both rated for -1 G). These helicopters can loop and roll, but what keeps them from sustained inverted flight(since they can handle one negative G)is the pitch range that the main rotor is capable of. Even if a (very brave) pilot wanted to he couldn't keep the aircraft upside down. However, just as the evolution of the airplane has proved to produce aircraft capable of inverted flight it will probably be just a matter of time before an engineer overcomes the limiting factors in todays helicopters. We will see...
Richie