Actually it is the slip stream effect. I've never actually ridden in a peloton, but I have ridden many a mile with other riders and indeed it is easier to pedal when tailgating another rider than it is to pedal while riding alone.
I used to ride with a pretty lady named Glenna, and while I did my share of infront miles it was a lot more fun to be riding right behind her. Awesome view!
And apparently the effect is much greater in a peloton. I have carefully watched closed course races and seen that in the peloton there is a constant flow of riders from the back to the front of the pack. Those at the front get tired and drop all the way to the back, so everyone gets a turn in the lead.
The TdF is actually a team event with individual rider prizes; one of the grand strategies is for a team to "protect" its strongest rider by forming a peloton in front of him and letting him preserve his strength for the dash to the finish line.
