From AP via CTV.ca:

Fox News Channel referred to Michelle Obama as "Obama's baby mama" in a graphic on Wednesday, the latest in a trio of references to the Democratic presidential campaign that have given fuel to network critics.

The graphic "Outraged liberals: Stop picking on Obama's baby mama" was flashed during an interview with conservative columnist Michelle Malkin about whether Barack Obama's wife has been the target of unfair criticism.

In the past two weeks, Fox anchor E.D. Hill has apologized for referring to an affectionate onstage fist bump shared by the couple as a "terrorist fist jab," and Fox contributor Liz Trotta said she was sorry for joking about an Obama assassination. ...

Joan Walsh, a columnist from Salon.com, criticized the graphic on Thursday as a slur.

"Do you try to explain that 'baby mama' is slang for the unmarried mother of a man's child, and not his wife, or even a girlfriend?" Walsh wrote. "Are they racist, or just clueless? Isn't there racism even in their cluelessness, if somebody didn't know what 'baby mama' means, but used it anyway? Even at Fox, won't somebody have to apologize?"

It's not in the above wire story, but apparently someone at Fox has apologized.

Salon also has this backgrounder on the offending phrase.