I also have a pitbull. He is two now and still pee's when he gets excited. (eg- when someone comes to visit) He has outgrown it somewhat. I would suggest teaching him to lay down and giving him that command when you notice the signs of excitement and peeing comming on. Before someone comes in the house I would take Kashus (my dog) outside to greet them just in case. Hopefully your dog will grow out of it as well but you will have to be a little creative until then to keep the house clean. :) hope it helps
You are probably mistaking "happiness" for "excitement". Many dogs, particularly young puppies, will urinate when they are excited. If you greet your dog in a "happy" excited manner, you will cause him to urinate.
Many times dogs and pups with low self-confidence with urinate out of submission. They're trying to tell you in the only way they can that they recognize you are the boss.
In both these case, do not greet your dog with a lot of talk and petting. Calmly ignore the dog until he clams himself down. No talking, no touching and no eye contact - just go about your own business.
If you haven't been to a dog training class, I highly urge you to go. Training is all about teaching the human to communicate with the dog, and yours is a pretty classic situation. You aren't alone by a long shot, and I hope you'll be one of the few people who will take the time to learn doggie communication skills. The rewards are enormous and you'll have an unbelievably great relationship with your dog if you go to school and do the homework. :-)