From an interview of JK, the CEO of Turner Broadcasting.

JK: I'm a big believer we have to make television more convenient or we will drive the penetration of PVRs and things like that, which I'm not sure is good for the cable industry or the broadcast industry or the networks.
CW: Why not?
JK: Because of the ad skips.... It's theft. Your contract with the network when you get the show is you're going to watch the spots. Otherwise you couldn't get the show on an ad-supported basis. Any time you skip a commercial or watch the button you're actually stealing the programming.
CW: What if you have to go to the bathroom or get up to get a Coke?
JK: I guess there's a certain amount of tolerance for going to the bathroom.
http://www.inside.com/product/product.asp?entity=CableWorld&pf_ID=7A2ACA71-FAAD-41FC-A100-0B8A11C30373