We've contacted Samsung about the problem where performing a mmc erase could hardbrick your phone (i9100, i9100g, n7000, m250 - MAG4FA, VYL00M, and KYL00M with firmware revision 0x19 // T989 and I727 with fw rev 0x12) if it's having a faulty emmc chip.
They're working as hard as possible on a clean solution which will be ready soon.
Please be patient and try to not flash any leaked kernels or kernels based on sources where MMC_CAP_ERASE is present.
This app http://forum.xda-developers.com/showpost.php?p=27014974&postcount=1
can be used to identify if your phone is affected or not.
CM9 kernels are safe to flash.
Update 14:56 CEST:
Patches will be out in form of new official ROMs and also sourcecode releases after testing, which might take some time.