Venge spotted at Mike's Bikes in San Francisco
But the frame is only a relatively small part of the aerodynamic picture. Far more important is body position. So here's photos of Cavendish during his two victories this year... first stage 5:
Then stage 7:
These photos aren't cherry-picked: they're the first ones I found on CyclingNews of the respective sprints in the final 100 meters or so. Just look at Cav's position: the top of his helmet is hardly higher than his back. Sure, some other riders can be seen with low head positions, but that's with their faces down, not sustainable. Cavendish is always looking forward, never down. He's watching where he's going.
Here's a shot from Mark in 2009 outsprinting Thor Hoshovd, Oscar Friere, and a Milram rider (Ciolek, I believe). Look how vastly superior his position is to his competitors:
It's often said sprinting is about raw power. Yet Cavendish's power is easily overestimated. It's been reported that Cavendish doesn't "test" well, and that in sprints he produces only around 1300 watts, a non-spectacular number. Of course, he's producing that power at the end of a long difficult race, after finishing with several kilometers over technical roads at close to 60 kph sustained. But the guys he's beating consistently can also put out power under the same circumstances. Cavendish wins not because of superior power, but because of superior speed, and except under the most exceptional circumstances most power goes into wind resistance.
Look at a Tour finish and the final 200 meters aren't enormously faster than the 200 meters preceding. Cavendish, for example, has Mark Renshaw, a man who's won Tour sprints himself, leading him out with his own maximal effort. No -- the sprint of a Tour stage, in addition to critical factor of position, is a time trial, and time trials are won by power balanced against aerodynamics. Cavendish is so unprecedentedly successful because he has one of the all-time great sprinting positions in professional cycling history... to beat him you've not only match his power but substantially exceed it.
The aerodynamic frame is icing. But the primary factor in aerodynamics is body position. 100% of great time trialists have great body position, going back as far as photographic history records. And Cavendish looks like a time trialist when he sprints. That he's able to hold that poise in the midst of so much chaos is remarkable.