The Rat

[megaloman]

so i dont know if i have missed all of yalls guesses, but my best guess is that ben's rat on the boat is MICHAEL. i fits, locke would be shocked if he was told (ben told locke he might want to sit down), all the comercials say "someone you thought you would never see again (we already saw walt). AND.... i sorta makes since because ben sent michael off on the boat, kind of putting michael in a state of "i-owe-u-one". the only thing that doesnt make since is why walt and michael split up...or did they? or maybe walt is with michael but can teleport the the island....
ok now im rambling
thats my guess, the rat is michael

[Dessie_hull]

i dont think it can be anyone else on the boat other than michael. Locke would only be shocked at someone we know. I think it was the monster taking walts shape when he spoke to locke saying theres work to do. just like when jacks dad told vincent to wake up his son because there is work to do

[megaloman]

oh ya i guess it was probably smokie taking the shape as walt