10,000
It's $10,000 within any 24 hour period. Of course I'm not sure how this can be of much relevance of income because it's not hard (if you are playing for some big money) to take a loss of a few thousand for the day but be left standing there with over $10,000 in chips on your person! You end up not being able to cash them all out that day is all but cashing out for $11,000 at the window does not mean you actually made $11,000. What you cash out for has no bearing on whether you gained or lost when it comes to chips!
I watched a foreign national stroll up to a cashier's window just in front of me with a large amount of chips once, maybe 35,000 or so. They started asking him for ID, etc. He had no clue what was going on, couldn't speak a single word of english, provided a passport and became visibly nervous upon getting a hard time from the cage... YOU AIN'T IN ASIA ANY MO' BOY! I don't know what happened because I ended up just going to another window and there was no explaining anything to him because of the language barrier.
The original question is yes. The IRS can tax anything and everything! If you get handed a 20 dollar bill from a little old lady that you helped cross the street back when you were a boy scout... this is taxable income!