The W-9 Form is probably just to get your SSN for a
Currency Transaction Report (sent to FinCen for money-laundering detection, not the IRS), which they are required to file for any cash transaction over $10K. Any reporting of your winnings to the IRS would be done on a W-2G or 1099-MISC by the casino, a copy of which the casino would give or send to you. But the casinos aren't required (and none of them do, afaik) to report poker cash game winnings (for US residents) to the IRS - only net tournament winnings exceeding $5K.
Make sure you keep a contemporaneous record of your cash game buy-ins, wins and losses, as described in the
US Taxes Sticky.