This message has greeted me every time I've visited this website for the past two years, and the string 'Mega XP' is noplace on the poll ideas page.
The discussion that provided an explanation took place in the Chatterbox 2.5 years ago. Now your hopes of ever finding an answer will be dashed as well.
Seriously though: XP = Experience Points -- those little things that we accumlate as our posts are upvoted, and fret about as our posts are downvoted. In the end, they mostly just signify how much time we've spent procrastinating our real $work. Mega XP would be a whole lot of the virtual stuff. And since the pollsters aren't able to hand out XP gifts, if you ever did have hopes of poll quest submissions inflating your XP your hopes will be dashed.
If I recall (it's been too long since I've set up a poll) pollsters do have the option of attributing a poll to you, the submitter. When that happens, you will see some XP (or loss thereof if the poll is negatively received), but not much.