Actually, it is typically much more difficult for a QB to learn the offense and the mental aspect of the game than it is to learn the physical skills of the game (think Feeley). A player working with a good trainer can gain an amazing amount of muscle mass in a matter of months. A good QB coach can make dramatic changes to a QB's mechanics in one offseason, as well. Intelligence is the one thing a coach can't change.
IMO, this makes Smith the best prospect to come out of the draft in a while. His intelligence should allow him the ability to learn the offense at a much quicker pace than most QB's. Smith also has all the intangibles...good awareness, good decision making, leadership ability, and presence. Give him two offseasons and he will almost certainly have the physical tools to go along with his mental skills. I believe that Smith would project well as a backup for his second NFL season, and likely fully capable of starting midway through his second season, if not being named the starter from day one.
Could he bust? Sure, but I would be willing to bet that he won't. Most players that bust have all the physical tools but are lacking in the mental aspect of the game, ie. Ryan Leaf. I believe you will find intelligent players fair much better in the NFL even if their physical tools aren't quite as awesome as a coach would like, ie. Zach Thomas. It is great if you can get a combination of both, but if not, I will take greater intelligence over raw physical potential any day of the week.