From this week's McLaren radio transcripts, I think both Hamilton and Button were supposed to go out again at the end of Q1 on options but as Jenson left the pits they were adjusting Lewis' front wing and soon realised that their times were good enough and they didn't need to go out so they kept Lewis in but it was too late for Jenson, who didn't sound too happy about it afterwards.
http://www.mclaren.com/2012/chinese-gp/qualifying?expand=1
(After their first run in Q1):
Pit » HAM: "Just in case we have to go out again in Q1 on the option tyre, what would you like us to change on the car?"
Hamilton: "Half a hole of front wing."
Pit » BUT: "MAS is out on the option tyre at the moment. He's just set two purple sectors, so we might have to run again."
(Button leaves the pits)
Pit » BUT: "The car behind is PER. He's just started a timed lap."
Lewis switches off his engine, he doesn't leave the garage after all.
Pit » HAM: "Some last minute information revealed that we don't have to go out again. VET and SCH stayed in the pits."
Pit » HAM: "We're okay, we will be through."
Button: "Did you not realise that Lewis wasn't going to go?"
Pit » BUT: "He had his engine fired, he was going to go."
In Q2 Hamilton only did an out lap on his 2nd run as his first run was good enough so he used those same tyres for his first flying lap in Q3. Jenson on the other hand did 2 full runs in Q2 and the team asked him if he could use the tyres from his 2nd run in Q2 for a flying lap at the start of Q3, but Jenson declined.
(Both cars out on their 2nd run in Q2):
Pit » HAM: "You're going to be okay. Box this lap, save the tyres."
Pit » BUT: "Keep pushing, Jenson, keep pushing."
Pit » HAM: "These tyres stay on. They've just done an out-lap, so I suggest we use them for our first run in Q3. That leaves a fresh set for the end."
Pit » BUT: "You're P9, Jenson. You're through."
Pit » BUT: "Do you think we can run this set of tyres again at the start of Q3?"
Button: "I don't think we should."