All Apple has to do is present paperwork to prove they have a similar warning to completely invalidate this lawsuit. It's common knowledge that any object left out (or used) in direct sunlight is going to get hot.
Not every warning is considered valid. A lot of EULAs and contracts (cell phones, etc.) waive your right to sue companies, but they aren't valid in court in many instances and the lawsuit proceeds anyways.
If the claimants can demonstrate that they - and numerous other iPad owners- had a reasonable belief that the device would function in the sun (a lot of iPad ads show the device outdoors), such as the fact that comparable electronics (eReaders, iPod Touch, iPhone, etc.) can be used without issue, then they have a decent case.