Hilton HHonors is offering 1000 bonus United Airlines Mileage Plus miles per two nights stay at all Hilton properties between October 1st and December 11th. You can earn up to 5000 bonus Mileage Plus miles. These are in addition to the 500 miles that one would earn having United as fixed partner on most full service locations, or 2 miles per dollar spend if United is variable miles partner.
You need to register for this promo and have United Airlines Mileage Plus either you variable or fixed airline partner. Bonus miles will be posted to your account shortly after the stay, based on my personal experience with Hilton, although the Terms and conditions of this offer say that it can take up to 6 weeks.