I thought Ben and Tom said that, since the "purple sky" event, anyone that left the island on the sub would have been unable to return. The purple sky event (EM Pulse?) made the sonar signal inoperable. Remember Locke getting the Dharma message saying that after he tried to activate the sonar on the Flame station computer?
There was something interesting that Juliet mentioned once, it seemed in jest. When Kate asked her what they were building with all those rocks that Kate and Sawyer were breaking and moving during their captivity, Juliet said that they were building a runway...for the aliens from space. If a balloon or helicopter can make it to the island perhaps a plane can land on it, with the proper approach.