The flight was originating in Tampa. According to the Tampa airport, all passengers were safety taken off the plane, and crews were able to put out the fire.
Shortly after it happened, passengers began tweeting. One with whom we spoke says everyone was OK, and that the United staff was very professional when it happened.
United released a statement, saying "Flight 1948 from Tampa to Houston today experienced a blown tire prior to take off. There were no injuries. Passengers deplaned via air stairs and were transported by bus to the terminal where we are working as quickly as possible to get them on their way to Houston this evening. We will substitute another aircraft to operate the flight which is now scheduled to depart Tampa at 10 p.m."