PhreakNIC is now over for 2015.
Your registration price is fully deductible as a charitable contribution to Nashville 2600 Organization (a charitable organization classified under IRS code 501(c)(3)). If you bought a current year's T-shirt, the fair market value has been determined to be $15.00, which must be subtracted from the price of your purchase to determine the donated value for the purpose of deductibility. Since shirts from previous PhreakNICs were sold below what could otherwise be considered fair market value, purchases of those older shirts are not deductible.
Additionally, if you volunteered any time at PhreakNIC, whether it be for setup, teardown, speaking, AV work, etc., your incidental expenses may be deductible. These expenses may include items such as any necessary overnight stays at the hotel, food expenses, and mileage driven to be there. It notably does not include your time, however. If you're interested in more details, please contact a qualified tax professional.