dog nipping?

Question
We have recently adopted a one-year old Terrier.  He seems to be adjusting nicely.
Our only difficulty with him thus far is that he wishes to put his open mouth on our hands (nipping??)when he gets excited.  We feel this behavior is inappropriate.  
We mostly see this behavior when we are putting the leash on him for a walk.  We have him sit before leaning to put on the leash, but he turns his head and gently puts his open mouth on my hand.  Why does he do this? And how do we get him to stop? We have tried a firm "NO", but it continues.  This morning, after telling him "NO", he repeated the behavior, so we walked away.
We also have seem this behavior when playing fetch.  He will drop the ball, but when we bend over to pick it up, he puts his open mouth on our hand. He also will do this when first let out of his crate.  We now let him out of the crate and wait until he has calmed before petting or playing with him.  This usually takes about three minutes.  
He has not hurt anyone, but we feel an open mouth does NOT belong on skin!
We would appreciate any advice you might give us in how to best handle this situation.  
Many thanks for your time and wisdom.
Mary

Answer
this is NOT a sign of aggression, it's a signal of overexcitement and the dog's inability to redirect it.  Your waiting to greet him after letting him out of the crate worked, and your ignoring (walking away) when he offers this behavior is also excellent and will work, given time.  Terriers are NOT retrievers!  He may be so excited with this play activity that he can't fully control his response.  This dog is brand new to your home.  It can take months for a dog to calm down, feel safe and be able to contain hyperactive reactions to interaction or noise, activity, etc.

Learn about positive reinforcement training and teach this dog (to start with) one behavior, using a made up word (because most dogs have learned not to respond to the "sit" command, as well as "come", "down", etc.)  Once he has 100% response to his new "trick", move on to another.  Add as many as you wish, one at a time, slowly.  Give him many ways to PLEASE you and many ways to control his own anxiety, over excitement, etc.  Give this time and patience.  IGNORE what you don't want, REWARD what you want.  Over excitement at the sight of a leash is NOT unusual at all.  Curb this by changing the cue: take the leash down and ignore the dog for several minutes; when you attempt to attach it, if he mouths you, yelp loudly and walk away for a few minutes and try again.  He will soon learn to associate negative results (no walk, just you disappearing!) with his own behavior.  Don't give up on him.