So I do not know (in the mathematical sense) that ChipKill won't create a problem, but I think it is unlikely.

In general, if the ECC infrastructure works (e.g. DRAM detecting bit corruption and reporting it to the OS), the odds of an attacker finding a piece of RAM that flips bits "just right" to bypass the checksumming without triggering a OS panic are low enough that I'd rather worry about software bugs.

DoS may still be a pretty viable attack, though.