B.R.
Totally normal... my daughter did it, too, but usually BEFORE falling asleep so when I would go in and check on her there she would be with her naked little bottom. The best cure- onesies. She was also sticking her hand in her diaper, and I just told her "no, yucky" until one day she came up to me with poop all over her hands- I put her in a time out, and after that I put her in timeout with no warnings when she would do it, and she stopped.
Also, don't make the mistake many parents make by deciding he is ready to potty train just because he can take off his diaper- the two skills are totally unrelated, and you can do a lot of damage, actually, but putting too much pressure on him. You can try, but you have to consider that he A) has to be able to tell you he needs to go, B) has to know when to "hold" and when to "let it go" (this is what my daughter is having a hard time with), and C) he has to be emotionally ready, ie he has to want to do it. If he's not, don't feel bad and don't punish or pressure him.