I Also have some cracks in the housing of my White iPhone 3G, bought on PAYG when we had the option. There a:
Crack between the simtray and bezel
Crack between the Sleep/Wake button
Crack in the bottom of the phone in the centre of the USB connection on the back.
I've had my phone in a Wave case for about 6 months but realized that was scratching the bezel, so then put it into a shades case for the rest of the time.
and ive just checked my receipt and i bought mine on 19/09/08 cr*p i didnt have apple care with the phone cos we dont have an Apple store where i live, and i only had 2 options of where to buy the phone, O2 or CPW, so i choose O2 but their insurance is questionable at the best of time so didnt get insurance from them.
So i suppose it will be ok, my phone apple tomorrow see what they say, but im not holding out much hope as it has gone past the 12-months, and i should have phoned them earlier when i noticed the cracks.