Well Jeff, you are right, it means the signer must attend for 1 full acedemic year. It does not guarantee playing time or a spot on the team. But the signer can not be recruited by other schools who are part of the national letter of intent program.
So per the commitment he's locked in to his first year regardless of what happens unless he decides to go elsewhere and not play. Whether the school he commits to would make him honor that is not clear, given his age, but for certain he has reduced his options.
http://www.national-letter.org/faq/
Interesting discussion anyway, at least more interesting than reading about MTD's awful airborne shooter block/charge interpretaion, isn't it?