I think some people in the surveys might be stretching the truth a little. 0.5 cf/min is a good SAC rate, not an average SAC rate. I suspect the real average based on how much gas my buddies have used on the same dives to be in the 0.6+ range. For new divers, the average could be 1.0 cf/min. Getting weighting and buoyancy under control will drop SAC significantly. I recall mine went from 1.0 to 0.8 instantly after I did a proper weight check and concentrated on staying neutral the entire dive. The improvement down to the 0.5 range was gradual as I became more comfortable in the water and learned to swim horizontally, streamline my gear, not use my hands to turn, etc. I frequently get below 0.5 now, but I would never plan on it.
I think it depends where you are and what you do. If you are in a drysuit, wearing a big steel tank and carrying 30 lbs of lead to boot, that takes effort no matter how good you are. If you are in the Caribbean diving in a T-shirt and shorts and wearing 6 lbs of lead and sitting watching the reef around you, getting under 0.5 isn't that hard for an experienced diver.
I am a fairly big guy, but in these conditions I have gotten my SAC as low as 0.38 (disclaimer: according to my Suunto). But as you say, I certainly don't plan on that...