One thing that's accepted as a given in D.C. is that there's nothing amiss when people constantly cycle back and forth between raking in big bucks consulting for private-sector clients and going on the air to share ostensibly independent political commentary.

While people constantly obsess about the revolving door between government and lobbying, this other revolving door -- between consulting and on-air commentary -- gets almost no attention at all, even though it's widespread.

But now comes a situation that's focusing a bit of attention on it: The case of two CNN contributors, Alex Castellanos and Hilary Rosen, and their work for BP.

This morning's Washington Post reports that BP has retained the services of Rosen, a Democrat who heads the Washington office of the Brunswick Group, to help out with BP's lobbying and public relations offensive inside the Beltway.

Rosen has contracted out some of this work for BP to Castellanos, a Republican consultant who's perhaps best known for the racially charged Jesse Helms ad showing white hands ripping up a resume.

In a statement, CNN says that neither Rosen nor Castellanos will be invited on the air to discuss topics relating to BP. "Both Alex and Hilary are contributors used primarily to comment on political issues, and they are not being used to discuss the oil disaster story," CNN spokesperson Edie Emery emails me.

That's all very well and good, but there's an interesting larger issue here: Should networks ever turn their airwaves over to analysts whose selling point to clients is that they have influence with lawmakers currently in power?

The question is whether consultants whose livelihood depends on maintaining good relations with lawmakers should ever offer political commentary on the networks, even on topics not directly relevant to their clients.

One analyst who's frequently on CNN said the answer is No.

"When contributors on the networks have agendas before the government they're analyzing, it is a blatant a conflict of interest," this analyst grouses to me. "The networks should not let contributors -- whether they're lobbyists or advocates -- analyze the White House and Capitol Hill on the air if they are simultaneously representing clients of any kind before the government."

I don't know if I'm prepared to go that far. But what's interesting to me is that it's simply accepted as a Beltway fact of life that commentators -- Republicans and Democrats alike -- frequently offer ostensibly independent-minded commentary on the government and politics even as they're also taking in big bucks representing private-sector interests before that same government. This is the revolving door that's never discussed at all.