True, Smith won't be handed the job from the get-go, but in order for him to have an opportunity to make an impact (sooner rather than later), he'll need as many reps as possible. One scenario is for him to arrive on time, receive 100% of the reps at LT during TC (Light doesn't need any time there), then a full pre-season vs NFL competition. All the while, Light is doing the same at RT. Evaluate both players throughout the process, with Nick as the backup at both RT and RG.
All draft prospects are dealing with the NFL transition, and all develop at different paces. If, as you state, his run blocking needs some work, he may have the necessary time to improve during this pre-season. We, as fans, can't put a timetable on the development of rookies -- it's individually based.
If, Smith isn't deemed ready this pre-season, return to the starting OL of '08.
Never thought I'd be speculating about the #3 pick in the draft -- twas fun...although I won't be counting chickens.