They learnt their lesson. Sadly, it was the wrong one...
It was working again - briefly - but now it's keeled over again.
I reckon that O2 ignored all the pre-registrations and just looked at the way the first launch played out - no queues (honest ones, not stage-managed ones) outside stores, low demand, slow pick-up - and figured that it would be the same this time round. Quite possibly they figured that although was loads of hype last time, there was ne real surge and that it would be the same again.
What's the chances that when the third iPhone comes out* they'll look at today's antics, figure "OMG We'd better spend bazillions on extra capacity to cope" and, since all people really wanted was 3G and GPS in the first place, there will be minimal demand again and they'll have wasted all that money and effort...
*Since Apple call each iPod by version, eg iPod 2G, Nano 3G, what will they call the 3rdG iPhone, since the iPhone 2G is already called iPhone 3G...?