I learned in Tae Kim's Grammar Guide that だ cannot be used when asking a question. But in the book Japanese Sentence Patterns for Effective Communication by Taeko Kamiya, I found several example sentences that used だ in a question. I also have seen it being used this way in anime.
I'm guessing there is some type of exception to all of this? Anyways, I hope someone can shed some light on the subject for me. :)
== EDIT ==
Here is an example from the aforementioned book:
どうしてゆうべ来なかったんだ? (Why didn't you come last night?)
And here is an example from the title of the 249th episode of the show Dragon Ball Z:
悟飯はどこだ!? (Where's Gohan!?)
Another odd memory is that he was provided with info on why "同じ" appears to break some rules, but his question to the world on that is still in the guide AFAIK.
– Claytonian Sep 08 '11 at 22:17