Kay I know that their is no official answer to this question, but I still want view the evidence and see which is the right answer by a poll.
Deidara is a man.....
1.the name is masculine.
Deidara is a woman....
1. Seems to be fem, even compared to other members
2. Hidan calls Deidara "Deidara chan" in chapter 332(could be an insult though which would make it pro male in that case)
3. In terms of Kishimoto behavior she's female. i.e. the actuall beating of an ally for comic purposes. I could be wrong, though I can't think of a male character in kishi's series that behaves like this, but Sakura and Tsunade are both known for these types of outbursts.
I can't really think of anymore proof that Deidara's male or for the female side, but feel free to add evidence that I forgot.