It seems that miles earned through spending money on credit cards are not taxable, because they are considered a reduction in purchase price. This means that if you spent $1000 and got $500 worth of sign-up bonuses, it just reduces your basis in the goods you purchased, in the form of a rebate.
However, no one on these forums, nor anyone else I was able to find online, addresses the issue of getting a bonus worth $500 when you only spend $1, like by the United/Continental/Starwood cards. As you only spent $1, and got much more than that in return, there is no basis to reduce.
Are these miles taxed? Any opinions?