I'll just reply to myself again (apparently talking to yourself is the first sign of madness, what number to you reckon message-boarding yourself is??):
According to PL website, "In keeping with UEFA guidelines, an Under 21 player is defined as one who is under the age of 21 on 1st January in the year in which the season commences. For the 2010/11 campaign Under 21 players will have been born on or after 1st January 1989."
Note the "under the age of 21 on 1st January", not "21 or under" - Naughton's DoB is 11 November 1988, so he has to be included.
U21s don't have to be registered, so they're all in.
In that case, I can't think of anyone who's been left out... obviously Woody, but 'Arry said ages ago he would be.