I got several emails this morning regarding this new offer for first timers with United Mileage Plus Visa. 50K for a $250 spend is about as good as it gets.
Link (link expired) for terms and conditions
You must do this by phone. Call 1-800-421-4655 and choose apply for a credit card option. As always get the name and employee number of the customer service rep. Make sure you get it in the conversation that it is for 50,000 points.