I'm not a CPA, but I've dug out my old tax textbook and found a 1987 U.S. Supreme Court case that decided on the issue of whether a professional gambler could deduct losses as a business loss. It's not cut and dried; it is possible to do so, and escape the limits that apply to gambling or hobby losses.
Commissioner v. Groetzinger, 480 U.S. 23
http://caselaw.lp.findlaw.com/script...0&invol=23
Groetzinger was a full-time gambler, and it was his only means of making income. After going at length on the history of defining a trade or business, the court (6-3) held that "(I)f one's gambling activity is pursued full time, in good faith, and with regularity, to the production of income for a livelihood, and is not a mere hobby, it is a trade or business within the meaning of the statutes with which we here are concerned."
The book I got this from is about twelve years old, and I don't live close enough to a law library to see if this has been changed, though I doubt it has been. Also, even if you meet this test but are doing on-line gambling from a place in which it is illegal, you might expect some difficulties because another provision of the Internal Revenue Code disallows deductions that violate public policy.
Hope this helps.