You can edit content for 15 minutes after you add it and then it's fixed. So in theory you could delete a question (remove the content) at that point, otherwise you can't.
The rationale behind this is:
1. People will answer your questions and if you delete it then you would also be deleting their answers. Clearly we can't let someone delete another persons content. We have discussed being able to delete a question before it is answered but it's never made the priority list (yet).
2. If you could edit or delete content after the fact then you would be changing the context of the "discussion" e.g. when someone refers to your answer or doesn't mention things you said to avoid repetition.
Hope that makes sense. We are always open to ideas/suggestions for making things better. I talk to the Gnome regularly (to the extent that guy talks) and as he likes to say: "It's great when someone ELSE comes up with a good idea now and then".