some other quick possible jacob theories:
1) that its a future desmond, trapped in a time loop
2) that its a future john locke, trapped in some sorta time loop (like desmond). that could explain why locke couldn't see him, and how they both "hate electronics"
3) ben DID make him up. but has the walt-like ability to imagine things and make them real -- possibly a power only granted to children, but locke grew up there, so the power never faded.
4) its that guy from the jungle who talked to ben as a kid, then told locke to find sawyer ... i think his name is richard. whatever the case, he doesnt age (sure, lets throw in time loop again)
