It was only a matter of time before the signup bonus arms race made it up to the next level. The United Visa is offering 21,000 bonus miles and even 25,000 bonus miles with annual fee waived the first year.
They took a page from a recent Citibank American Mastercard offer, though, and are requiring $250 in spending on the card before awarding the bonus instead of crediting it after first purchase. Still, an extra 5000 miles award for spending $250 seems quite worthwhile.