Whenever someone says “guess what”, it sounds to me like they are asking a question. I know I’ve seen it in print that way in dialogue, as in “Guess what?”
But yesterday my daughter, in the 4th grade, was corrected by her teacher for using the question mark. She said it’s an imperative sentence and should have a period, which I suppose technically makes sense, but I’m curious because it doesn’t look right to me. What do you guys think? It seems to me when I say “guess what?”, I’m not so much telling you to guess, but rather asking if you are able to guess. Or am I way off on this?
thanks for the link, but I disagree with its assertion that “guess what” starts flat and stays there. I think when I’ve heard, and used, that expression, there is a often a definite interrogative sound to the way it is used. It’s often used with a certain measure of shock or surprise, as well.
It’s all about context – there is no covering rule that can be applied here in all situations. It’s much like the following:
[ul]
[li]“It’s raining.”[/li][li]"It’s raining?[/li][li]“It’s raining!”[/li][li]“It’s raining …”[/li][/ul]
We can all think of situations and speech patterns in which each of the four examples above make sense.
Strictly speaking, yes, the teacher is right - you’re telling somebody to guess what’s up.
But I would probably have written it with a question mark, too, until you made me think about it logically.
I think this is one of those cases where the specific convention totally trumps the general convention, though (ie., everybody writes “guess what?” with a questionmark, and will continue to do so)
Chicken Butt 
You know someone was going to do it…