He's an excellent choice, and far and away the best out of the other rumored contenders (John Krasinski aka Jim from The Office, Ryan Phillipe who's about 5'8", Channing Tatum *shudder*, Mike Vogel from Cloverfield, Chance Crawford from Gossip Girl, etc) Of this group, he's the only one who has the look and the acting chops (see the abovementioned Sunshine for proof) to really pull this off. Plus, he's got a potential break-out movie in Scott Pilgrim Versus The World (which is supposedly looking FANTASTIC) coming out this year that should raise his profile and make Cap easier to sell.
Cap needs to have the range to pull off nerdy, pre-transformation Steve Rogers, transform into a believable battlefield legend, and match Robert Downey, Jr. toe for toe and make us believe that Tony Stark would shut up and follow him into battle in the eventual Avengers movie. Evans is a great choice, so long as they avoid the temptation to re-write Cap to be more snarky or sarcastic because it fits more with the "type" Evans traditionally plays.