I am no expert by any means but until they get here...
You will actually GAIN lift when you build some forward velocity. A really heavy helo migh actually only be able to leave the ground through using a take off roll. This is since the airmass available for the rotor to accelerate will increase. Instead of the volume of air being rotor area times rotor downwash velocity it is rotor area times the resultant between downwash velocity and forward velocity... er... a picture would really help here I guess.
During an autorotation, 60 kts or so of forward speed is essential since you actually flare before landing (or impact...), using the airspeed to kill the sinkrate. Low and slow is a death zone where autorotation is impossible. Having some airspeed built up before leaving the altitude where falling straight down is survivable if the engine decides to call it a day is desirable.
You'll also want some forward speed on approach to avoid creating vortex rings, basically a ring of rotating air around the rotor system giving the rotor very little chance of accelerating the air further to create lift. I'm not sure about the english term but a direct translation from Swedish would be "sink-through". Very dangerous phenomenon.
Cheers,
/ft
Edited due to incoherent grammar caused by serious lack of caffeine while posting.
[ 11 July 2001: Message edited by: ft ]