It looks like the IOC banned them at the 2000 games, I was wondering if this was going to fly with the IOC. I think they said it was considered "unsafe". Here's another article on it:
http://www.brianmac.demon.co.uk/articles/scni5a8.htm
...Among them was Michelle Jones, winner of the silver medal in the first ever Olympic triathlon. The hypoxic tent system, which is manufactured by an American firm run by the former British Olympic cyclist Shaun Wallace, costs £4,000. Despite this hefty price tag, it is becoming increasingly popular with endurance athletes looking for a legal way to increase their red blood cell count in the hope that this will make them run faster. Demand for the tents is reported to have surged since the introduction, at last year's Olympics, of a test, which could detect the banned hormone erythropoctin (EPO).
Among others who use hypoxic tents regularly are Britain's world cross-country champion Paula Radcliffe, England's former world triathlon champion Simon Lessing, Australia's ironman triathlon champion Greg Welch, New Zealand's Commonwealth cycling gold medallist Lee Vertongen and America's Olympic swimming silver medallist and world record holder Ed Moses. "Runners have a saying, sleep high, train hard - and that's what I do," says Suzy Favor Hamilton, America's top ranked 1,500m runner, who also uses the tent.
Here's another manufacturer that make a portable one:
http://www.go2altitude.com/
I would think that the tent would get pretty hot and stuffy inside.