This dead horse needs one more beating: I read a touring journal by a guy who biked cross country. He was having knee pain until switching pedals to plain, cheap, BMX-style platform pedals:

http://www.crazyguyonabike.com/forum...ssage_id=76745

According to him, having the ability to move his feet around on the pedals resulted in less pain from repetitive motion, but only a "very very slight" decrease in speed.

I've always used plain platform pedals and I will be riding a century in 4 weeks using them.