First, you'll have to produce photo identification, or the casino will hold onto the money until you do. If you show identification but decline to produce or declare a valid social security or other tax identification number, then 25% to 30% will be withheld depending on whether the jackpot is more or less than $5,000, and whether you are from the United States or a foreign country with a reciprocal tax treaty.
As of 2011, such countries were Armenia, Australia, Austria, Azerbaijan, Bangladesh, Barbados, Belarus, Belgium, Bulgaria, Canada, China, Cyprus, Denmark, Egypt, Estonia, Finland, France, Georgia, Germany, Greece, Hungary, Iceland, India, Indonesia, Ireland, Israel, Italy, Jamaica, Japan, Kazakhstan, Kyrgyzstan, Latvia, Lithuania, Luxembourg, Mexico, Moldova, Morocco, New Zealand, Norway, Pakistan, Philippines, Poland, Portugal, Romania, Russia, Russia, Slovakia, Slovenia, South Africa, South Korea, Spain, Sri Lanka, Sweden, Switzerland, Tajikistan, Thailand, The Czech Republic, The Netherlands, Trinidad and Tobago, Tunisia, Turkey, Turkmenistan, Ukraine, United Kingdom, Uzbekistan and Venezuela.
I've been trying to figure out the rules exactly, but it is giving me a headache. Please refer to IRS rules for issuing a W2G form for more information.
My thanks to Marissa Chien, co-author of Tax Help for Gamblers , and MathExtremist for their help with this question.
This question is discussed in my forum at Wizard of Vegas.