Mark has penned an excellent post addressing concerns about the lack of Ethernet on the MacBook Air, specifically to how it relates to the environment on the main Cornell campus.

Here's the crux:

It's worth noting that most laptop users EVEN NOW aren't bothering to plug into wired Ethernet, even when they HAVE the requisite port on their laptops. I strongly suspect we've already seen a large chunk of the hit on Red Rover's capacity from the shift to wireless, and Apple is just giving in to reality by leaving out a port no one is using.