Now this bit doesn't make sense to me. There are lots of situations where your knees can take a huge loading, far more than what they'll get on fixed. Take speedskating, for example, there's a lot of leg force involved in every stride, far more than your own body weight. You don't get knee problems with skating any more than with cycling.
I think the whole urban myth of fixed and knee injuries has mostly come about for a couple of reasons:
* People just starting with fixed. Riding fixed is quite a bit more work than riding geared, so without realising it they are suddenly adding a big increase to their training load, still trying to do the same rides/mileage. No wonder knee problems crop up.
* New bike setup and cleat position. Together with the above, these are the biggest causes of knee problems. Knee issues are common with any new bike, geared or fixed, no wonder people mis-attribute this to fixed.
The consensus mentioned above comes from some pretty serious fixed riders, LEL and PBP finishers no less, so I'm not a believer in fixed equalling knee problems.