All you need to know:
http://www.dansdeals.com/archives/38212http://www.united.com/web/en-US/content/mileageplus/awards/travel/types.aspx Click on "United air travel award rules:"
"You may redeem miles for travel in United Global FirstSM, United First®, United Business® or United BusinessFirst® , even if the space is not available. In these cases, United Economy in the same award type must be confirmed, and the front cabin will be waitlisted. For example, a Saver United First award must be confirmed in Saver United Economy to be eligible for waitlisting. If the courtesy waitlist does not clear, it will expire 24 hours before itinerary departure, and you will automatically be added to the airport upgrade standby list upon check-in. In these cases, you will be confirmed on a space-available basis by a United airport representative at the gate once the flight has closed for check-in. For United BusinessFirst and United Business travel awards, the difference in miles will be refunded when the United BusinessFirst or United Business class segment of the trip could not be confirmed. Waitlist requests may only be made over the phone with your local United Reservations office"