Walt in Season One, Season Two, and an artist's rendering of his projected appearance in Season Six.
The answer was to write Walt and his father, Michael, out of the show, which came to fruition when they boated into the wild blue yonder at the end of Season Two, leaving behind a whole pile of unanswered questions.
In what is widely regarded as the most retarded of the fan theories, it has been speculated that Walt and Aaron, the recently born child of Claire, are both magical children and will prove to be the countering "dark" and "light" forces of the island. At first blush, the theory hangs together perfectly, especially since the black/white contrast has been prevalent throughout the show's run. However, we have since found a snag in this theory: through some diligent research, we have come to discover that not all black people are evil by default.
Also, Aaron's just a baby, and while pre-teens may not be the toughest sons-of-bitches in the world, if this struggle comes to pass, we're laying heavy money on Walt simply punting Aaron into submission.
Others think Walt can merely astral project himself (meaning he can have out-of-body experiences at will) as part of his magical superpowers. Though this does little to explain why his voice is a full octave lower with each appearance.
We're sorry, but if you were hoping for a non-magical answer to this one, you're probably out of luck. If you were looking for all your answers to come from sound science, try some other show, like CSI. OK, bad example.
Will They Tell Us?
You'd better hope so, because Michael (played by Harold Perrineau) will be back for Season 4, and if Walt doesn't come with him, we can all look forward to another season of Michael looking disheveled and shouting "Walt!" several hundred times.
Of course, Walt returning doesn't in any way guarantee he'll bring answers with him. Don't be surprised if the writers spend all their time trying to explain why the supposed 10-year-old grows a fuller beard than Jack.