Um, I just realized something.....Walt totally has to get in on this time-travel stuff at some point so he can "drop in" on Shannon and Locke.
Nah, I'd like to believe that wasn't due to time travel
It's sort of 50/50. On the one hand, there's nothing to stop something *else* mysterious from also having been happening.
On the other, Locke explicitly mentioned that Walt looked older. Plus it would be really neat to realize we'd been seeing this time-jumping from the other end all along.