I ride a compact cervelo and what I have read people claim for compacts is that the rear triangle is smaller, and thus if everythinge else is the same is stiffer. I have also read that the long seatpost aids in dampening vibration, since it can "flex" a little.
What do you guys reckon? Any truth in it?
And that's why marketing is so much more fun than engineering. There must be some real brainwashing going on to convince people that they need a stiffer frame and a flexier seat post, and that these conflicting atributes aren't going to cancel each other out. All else being equal a compact frame will be less stiff for typical loading (weight at the saddle, reactions at the rear axle and head tube). Fortunately for the suckers, the in-plane deflection is pretty much negligible for both shapes.
As far as trends go, compact frames aren't going to look nearly as dated in ten years as Dura Ace cranksets.