There's a cheery website called "Dieoff" that does a pretty good job of factually presenting the worst case interpretation of demographic, economic, and resource consumption data. A contributor to the site, Richard C. Duncan, Ph.D., is the nominal (he acknowledges the contributions of many others) originator of the so-called "Olduvai Theory" which posits that we're quickly headed for a "post-industrial stone age."
The metric used by Duncan is per capita energy use. It seems to be a reasonable idea - mankind's ability to convert energy at ever-increasing rates has gone hand in hand with increasing standards of living (at least in those societies able to capitalize on it). And ranking of countries by primary energy use looks pretty similar to a ranking by standard of living, though there are exceptions at the high end.
Duncan cites a variety of sources indicating that per capita energy use peaked somewhere in the 1973-1979 time period. It's estimated that peak per capita energy use was about 11.15 boe (barrels of oil equivalent) or 6.46*10^7 BTU. He predicted that per capita energy use would decline at a rate of about 0.33%/year after that. Such a decline would lead to a 2008 per capita use of 10.47 boe or 6.07*10^7 BTU. What's happened?
Using BP's incredible site, the historical data spreadsheet gives me the data I need (by the way, anyone interested in energy, oil, etc. should spend a lot of time on that site). In 2008, per capita consumption was about 6.68*10^7 BTU. This is actually down from a peak of 7.24*10^7 BTU. The drops in 2007 and 2008 were quite steep.
Since the U.S. uses about 25% of primary energy and about five times the average, our effect on such statistics is huge, and we entered a deep recession in 2007. It remains to be seen what trajectory our recovery, if it comes, will take. For this reason, I don't think the current data support the Olduvai theory, at least at the present time.
It's also opined that the developing nations' striving for increased standards of living will overwhelm the developed world's (and particularly the U.S.'s) efforts at efficiency and I think that this is where the constraint will manifest. Since we use about 32.7*10^7 BTU per capita per year, a little less than five times the average, it's not likely we can find a way to bring the rest of the world to our level of use. And this takes no account of any peak oil consideration.
I've included a graph showing world population, total energy use, and world per capita annual energy use from 1980 through 2008. Click on it to see a larger and clearer version. Note the precipitous drop in the latter two categories in 2007 and 2008.