I don't know, each brand seems to have something for each season that is a must have for lovers of the designer/brand. Right now I can't think of any one bag that I have seen in the mags.
The last one I remembered being seen all over was the lv murakami in white on celebs. I think jessica simpson went nuts over it on her old show. But now I agree with everyone else: choose your own it bag!
I was reading that the YSL Downtown is becoming an It Bag since lots of celebs have been photographed with it recently including Salma Hayek, Giselle and Kate Moss...