if you look at the pics of her leaving Bergdorfs and zoom in, it looks like it could be a Cartier Baignoire in gold/diamonds??
http://www.imnotobsessed.com/2010/12/20/katie-holmes-and-tom-cruise-take-turns-with-suri/
http://www.imnotobsessed.com/2010/12/20/katie-holmes-and-tom-cruise-take-turns-with-suri/