IMO, that's also not what Greenspan was saying.  He didn't say anything about using energy more efficiently; he was narrowly addressing the impact on growth and inflation.  That's a far simpler point that is the issue of how hard it would be to further reduce the economy's ovetrall oil intensity.

"And since we seem to have been remarkably resistant to behavioral changes, presumably we'd pretty much have to find the transportation part in increased vehicle fuel efficiency, or else economic contraction if we can't get efficient fast enough.  I don't think that's going to be easier at all."

If our choices are reduced miles (via contraction) or reduced miles per gallon, then yes, it will definitely be hard to get an additional significant improvement.  But we're not limited to those two choices, as we have non-petroleum fuel sources: Ethanol, biodiesel, and electricity generated 95% by coal, hydro, nuclear, etc.  Those are very small contributors to our transportation needs today, but that will very likely change in just a few years.  

The jump to electric vehicles will be especially notable, since they completely shift marginal (meaning per-mile) transportation energy demand from oil to non-oil sources.  I think that with Mitsubishi selling a Colt EV in 2010, as they claim, we'll see people jump very quickly from gasoline vehicles to electric cars, at least for their commuting and around-town vehicle(s) in >1 car households.

In general, I think we're headed for a much more diversified energy infrastructure, right down to the consumer level, with a mix of gasoline, diesel, ethanol, biodiesel, conventional, hybrid, plug-in hybrid, and all-electric vehicles in the field for a long time.

Please don't think I'm arguing with you, Stuart.  I think you're doing a great job with these analyses, and you're providing a much needed service to the readers of this site.  Even though I think it's VERY important that we use statistics to understand our past and present situation, I think we also have to be careful about how we use that information to extrapolate into the future.  The real world has a very nasty habit of not following even the most logically and meticulously constructed, and best researched plans.  (And I have the scars, both metaphorical and literal, to prove it!)