Rewards and discounts issued by credit card companies and frequent flyer miles are not considered taxable income. However such rewards and discounts can reduce the cost of a tax-deductible expense.

In 2002, the Internal Revenue Service announced that it would not pursue the question of whether frequent flyer miles, and rewards and other promotional discounts awarded by credit card companies to cardholders.

This position was made public in Announcement 2002-18 (pdf, 1 page).

The IRS further explains in Publication 17 that rewards are taxable only if you provided information in exchange for the reward, "If you receive a reward for providing information, include it in your income." Generally speaking, rewards and discounts from credit card companies are not in exchange for providing information, and so would not need to be reported on your tax return.

Other types of rebates similarly are not included in your income. Further in Publication 17, the IRS explains that rebates on utility services are not income.

Rewards or Discounts Reduce a Tax-Deductible Expense

Goods or services purchased with credit card rewards or frequent flier miles would reduce your cost basis in the goods or services.
For example, if I buy a laser printer for my business costing $499, and I used a cash reward from my credit card of $50 towards this purchase, then I can deduct only $449 as a business expense.