As far as the 49ers go, they've got 2 very solid corners in Nate Clements and Walt Harris.  However, Harris is 35 and not long for this world.  The general belief seems to be that Smith's biggest downfall is that while he can play safety and corner he hasn't become excellent at either of them.   With Clements getting a lot of one-on-one assignments against the other team's best WR, the other corner gets a little more help from the safeties and nickel backs depending on the defense.  So he doesn't need to be super fast, but more of a physical guy.  Also, the 49ers signed Allen Rossum but he's getting a little old, so I'm hoping Smith gets some work as a kickoff and or punt returner.  I've read he did that at Oklahoma, but how good was he?

I think in general, the 49ers have a very solid secondary so Smith will not likely be an impact defender right away.  Rather he's going to get the chance to learn from Pro Bowlers while the 49ers try to figure out where he fits best.