It seems the Child’s pay ad from MoveOn was too much for CBS, and they claim they won’t run it because they don’t run “issue ads”.
Now, it seems CBS has run issue ads before: DaveNet : Is CBS Lying?.
MoveOn has posted the following information about the CBS refusal to run the ad:
CBS is refusing to run the winning ad, “Child’s Pay,” on the Super Bowl. Watch the ad and join the nonpartisan MoveOn.org effort calling on CBS to run public interest advocacy advertising like ours. Even if CBS won’t air the ad, we’re not going to let that stop us. During the Super Bowl half time show, at 8:10 and 8:35pm EST switch over to CNN to watch the ad CBS won’t air. And let us know you’re joining the one-minute boycott
[Update] American Samizdat has a good collection of links to articles about CBS banning the ad.