This is not directly related to the original post, but something Tanza said made me wonder if my issue wa related. I got Dexter when he was 8 weeks old, and at first we didn't have a problem with rough playing. However, within the last two weeks (starting around the time he made 12 weeks to the present) he has begun playing much rougher. I stop play when he gets too rough and won't quit (He stops when I say NO and licks me instead, but starts again), but it is very often and he isn't really doing it less. He plays too rough with Lola too (she allows it though, you could probably do anything to Lola and she wouldn't mind), as he bites her skin and shakes his head.
Possible this is part of the reason. Also, I would have to day that he is testing you now after being in his new home a few weeks. And remember, the older they get the rougher they play. Remember to, correction one time is not going to make much of an impression, it is being consistant that they finally get the message.