It's Ben Whishaw!
Probably best known as the new Q as of Skyfall.
I like this choice! I agree that as much as I love Colin Firth, his voice WAS a bit too mature for the young bear. When he was first announced, I had to do a bit of mental stretching to associate his voice with Paddington.
But (as much as I was rooting for Tom Hiddleston to be the one), I think Ben's a good fit. He's an adult male (as all those who have voiced Paddington have been), but he can sound youthful, innocent and earnest.
(Can't help but giggle at the fact that Ben seems to be tied to the name "Bond", though.)