My 18 month old son has been doing one nap since about 11 months.  He goes through phases when he goes down easy, and then there are times, like today when he gives me the ultimate run for my money. 

we go through his routine, book, some milk, and rocking in the chair.  then I put his "fish" on and lay him down.  sometimes he lets me do this without a peep and falls asleep easily.  Today he cried briefly when I left but then calmed down and talked to himself for a while.  half an hour later he was crying and calling me.  But if I go up he just wants to play.  He'll ask to be rocked but then will start asking for books or milk or just starts getting squirmy and tries to get down.  Or he'll ask me to hold hid hand in the crib but then he turns it into a game.  I hate leaving him crying and I know he is not trying to manipulate me on purpose but I feel so awful!  Right now it is 1:45 and he is still awake.  He was yawning and rubbing his eyes at 11:30.