We are seeing that more and more. It's worst in high heat applications like road course cars and after a while leads to coil failure and misfires. Often they look a bit rusty even with no moisture present around the cracks. We see it in early model coils as well as some Lotus engines so it is not a model year specific issue.
We just replace them when they go bad. Many cars ahve coil heat issues, I know the EVO is a good example. Ours hold up well most of the time but unfortunately coils can be considered a wear item so they do fail sometimes. Fortunately it's relatively uncommon and they last a long time generally.
Matt @ Monkeywrench
2001 Celica LSR car 823whp 2ZZ @ 41psi / 10.9@140 at 28psi / 202mph @ 25psi in the standing mile
2000 Celica GT-S 412whp with MWR turbo @ 16psi and stock engine / 550whp with MWR turbo @ 26psi on 93 octane - 11.5@129 on 93 octane
2005 Lotus Elise V6 295whp NA / 360whp 3.5psi SC
2001 MR2-S turbo - 508 whp - 10.85@130