I recently got To Catch a Thief on blu-ray, replacing my older laserdisc of it. It is a stunning transfer; the scenery really comes alive impressively. Oddly, the only flaw I saw was some moire on Grant's shirt in one scene (blu-ray isn't supposed to be susceptible to moire, since there's no comb filter anymore).
What was even more interesting was the coloration in several scenes. I'd never noticed on the laserdisc, but several scenes were lit by very green lights, and that definitely changed the feel of the scenes.
It's a fantastic movie, though, almost from beginning to end. I wasn't real keen on a couple of shots Hitchcock put in to add more drama or confusion, but they were mostly ignorable. And that's really the extent of the flaws. I really love Cary Grant (he was the reason I bought the movie, originally), and Princess Grace was also excellent.
I will definitely be watching it more in the future.