UA632 ORD-TPA 13 APR 11 (ECONOMY)

Arrived on UA7654 from Boston, this is the second leg of our trip to Tampa.
Headed to the gate to eat our breakfast bagels and while away the hour and a bit before we were due to board, our 757 was already there.
Boarding was called and a full flight was ready to go for the 9.30am departure time. The captain advised a 2 hour flight time, and we'd be away once some 'scheduled maintenance' was complete.
Roll forward half an hour... maintenance had discovered a hydraulic leak so the aircraft was out of service. Everyone had to get off. Quite why they hadn't discovered said leak in the 2 hours or so that the aircraft had been standing there before we boarded I don't know!
As it was another 757 aircraft was available and we took off/arrived 2 hours late. I guess we were lucky that O'Hare is United's hub airport and there were plenty of 757's around! To be fair the delay was dealt with pretty well.
Unremarkable crew who were a little more friendly than the usual UA bunch. Just a drink served.