Something similar happened to my girlfriend's Nokia 6100, when we flew to England in the summer. When we got back it wouldn't hold a charge, I took the cover off and there was a 3mm bulge in the battery pack!
It is well known, that the change in pressure can cause problems with the batteries, especially if the device is turned on during the pressure change - as most people probably put their iPhone into flight mode and not switch it off, before take-off, it probably isn't that surprising that they somethimes expand enough to crack the glass and possibly burst into flames.