I'm also disappointed that Katherine Heigel has been cast as Stephanie. I've seen photos of her recently as a brunette, and she doesn't look right at all.
BUT, I was also totally disappointed that they cast Renee Zelleweger as Bridget Jones, and I thought she did a great job. I loved both of those movies, even if they weren't quite like the books. Although Renee had Colin Firth and Hugh Grant as her supporting cast so that was a big help!