You can put anything you want on your tax form as a Profession, but the IRS (or the Tax Court if you have the funds to chase this option) will determine if they allow what you put, no matter what you think.
You must do MORE than simply list Professional Poker Player on the IRS Form for it to actually be accepted. You have to meet a number of criteria and the burden of proof for doing so is on the filer. The IRS seems to challenge people's professional gambler status frequently.
The good news is
The Department of Labor does officially recognize Poker as a Profession (Professional Poker Player) Therefore, Professional Poker Player is established as a legitimate defined and codified vocation by local, state and federal jurisdictions.
Last I saw, Professional Poker Player was given a vocation code (Standard Occupational Classification Code) of #27-2099 (Entertainers and Performers, Sports and Related Workers, All Other). It should be noted that Professional Poker Player is
not a Professional Gambler for tax filing or in any other similarity to gambling.
If you do file as a Professional Poker Player you will also have to pay the 'employment taxes' (Social Security and Medicare payroll taxes) and file quarterly for estimated tax payments.
If anyone has interest in filing as a professional, I am sure your accountant will show you the taxes for the 1st year you intend to do so on a Professional v Non-Professional basis. I know several Pros that went with 'Non-Pro' status because the Pro Tax rate ended up being much higher than Non-Pro even with deductions.
The following is a short definition and an article which is a good read dealing with taxes and poker.
Understanding Poker, Taxes, and Staking
In this article, we offer an overview of what poker players, at a minimum, need to know about their tax liabilities.
http://www.onlinepokerfaq.com/guide/tax-professional.html
It is possible to file as a professional poker player. As a professional, you can claim your net win as business income using Schedule C. This avoids all the costly problems caused by adding gross wins to your income and then deducting losses later. Also, a professional poker player may be able to classify as a business expense certain costs incurred as part of his professional activity. For offline players this is typically travel expenses; an online player could make a reasonable argument that reliable high-speed internet access and money transfer fees are costs of doing business.
The catch is that the IRS holds professional gamblers to a very high standard. If you have other sources of substantial income, or if you haven't posted wins for the last several years running, there is a good chance that the IRS will reject your claim of being a professional. Even if they accept your status, they may deny your expenses or consider your records to be deficient.
Another negative to filing as a professional is that you must pay 15% self-employment tax on your profits. This might be reduced if you are also employed in a regular job, but then your case for being a professional is weaker.
A good reference on filing your taxes as a professional gambler is the recent book Tax Help for the Frugal Gambler by Marissa Chien and Jean Scott.