It's true that anyone can call Jesus "Lord" and that would not necessarily make them a christian, but from the standpoint of recognizing who Jesus really is...I think it is very important that the disciples called Him Lord. Just like it was an eye opener for Thomas...
John 20:28 Thomas said to him, "My Lord and my God!"
Bingo! The guy finally got it after all that doubting. :)
Ok has anyone found out whether Judas called Jesus Lord anywhere in the bible yet? edit: Master and Lord are not the same thing? Does the Greek say it has the same meaning? Looks like it is.