Whenever I have this dream now, it isn't a bad dream.
I walk into class (or work, or practice, or the bridge of the interstellar battlecruiser of which I am bridge crew, or the room in which my cell of the resistance is meeting, or whatever. I didn't say it had to make sense - it's a dream). The other people are kind of surprised for a moment, then they all realize that nothing relevant is different from usual and continue with whatever they were doing. Everyone notices, and no one minds. Occasionally someone will comment that they had always wanted to go naked but never had the nerve. Usually, though, no one else undresses. When anyone else does, they fade into the background quickly.
I am quite sure that this is the same dream as those that others are speaking of here, since the change occurred over time, gradually. My dreams of only wearing underwear to class in second grade were KILLER. Okay, they weren't as bad as my WWI dream with the bloody sky and preternatural knowledge of death and the unfairness of war... but I digress.
A few characteristics still remain. First, it isn't a sexual dream. Occasionally sex will come up as an intellectually mediated topic (i.e. I don't get worked up), as people mention in surprise how nonsexual the situation is. Second, the plot is usually deadly boring, while the plot of my sexual dreams is almost always enough to keep me interested even if the sex were excised. Lastly, there's that classic opening frame you just can't miss: "oh, what? I'm naked?"
What's the point? I guess it's that this dream doesn't have to be a trauma. Of course, if you're traumatized by your own nudity, it will be.
Sadly enough, I wore pants while noding this
Update: I had this dream a while back, and it was a bad one like the typical case. I wonder what changed? Well, it hasn't recurred.
Update (2013): I now sometimes have to remind myself that I really did wear clothing to work every single day. And at school, too. No exceptions.