Ann Friedman makes a strong argument for the term "lady" in The New Republic: it "encapsulates the fundamental mutability of modern feminism" and occupies the middle ground between being a girl and a woman. (Props to Britney.) Moreover:
..."lady" splits the difference between the infantilizing "girl" and the stuffy, Census-bureau cold "woman." (Both still have their place — just not in the witty conversation that young feminists want to be having.) It's a way to stylishly signal your gender-awareness, without the stone-faced trappings of the second-wave. It's a casual synonym for "woman," a female counterpart to "guy," commonly used in winking conversation between one in-the-know woman and another.
Friedman acknowledges the tongue-in-cheek sensibility at play when most ladybloggers use the term, which resonates strongly; at this point, I have a hard time typing the word unless I'm mocking someone who doesn't treat women as equals or an article based on negative (and dumb) stereotypes. A headline from a recent example: "Ladies, You Are at Your Ugliest at This Exact Moment."