Though Kate Bosworth and Katie Holmes were also in attendance at Tiffany & Co.'s Blue Book Celebration, all eyes were on Biel. The pièce de résistance? Her stunning Oscar de la Renta gown. The look was rounded out by Christian Louboutin pointy-toe pumps and Tiffany & Co. jewelry, and we simply can't stop staring at it. With a down-to-there neckline and thigh-high slit, the dress is undeniably sexy. But, its simple silhouette — plus Biel's understated, elegant accessories — kept it utterly sophisticated. In case there was any doubt, this is how you do a deep V. Bravo, Biel.