Yep. And Google will send you and the IRS a 1099 form at the end of the year just so you don't forget.
every country has its own tax law. usually if you make an income, you are supposed to pay tax on it. if you are in a foreign country, you can ask a tax consultant who can tell for sure
I've ask my accountant. Answer was - first 40-50k$ annually we can easily hide as your business spendings and overheads. Now is my turn to bring these money
Taxes for AdSense Publishers - Though this article is for publishers based in India, the points are applicable for most countries.
The government impose taxes upon the income. So no consideration that from which business you got the income.