As Joshua Marshall reported initially, a rather innocuous spot produced by the United Church of Christ to advertise itself as a church tolerant and welcoming of difference has been rejected by NBC, CBS and UPN because it is "too controversial."
Jerome Armstrong at MyDD explains further that various cable networks owned by Viacom, among others, will be running the ad. Why? Until we hear otherwise from the UCC, my best guess is that the broadcasters are running scared from the FCC due to its overzealous and capricious crackdown on "indecency," as I have discussed before.
Since cable networks are not covered by the FCC, they have a little less to worry about. They seem to have made the calculation that while the FCC might step in due to even a few complaints from rightwing groups like Focus on the Family, it is less likely these groups would be able to mobilize sufficient numbers over the UCC spots in order to influence other advertisers to pull theirs.
In other words, the religious right has already achieved the victory they desired: Getting the broadcasters to self-censor due to the merest threat of FCC intervention. Thus it is no longer a First Amendment issue since no governmental action occurred, leaving no legal remedy to fight the networks' decision. (Free speech lawyers, please prove me wrong.)
Postscript at 12:35pm: Go check out Pudentilla's discussion of this as well. She makes some good, pragmatic points on this issue along with her usual witty dose of righteous outrage.