Even without this type of PCI attack, you still have to worry about a cold boot attack. This is were the attacker quickly power-cycles the computer to boot into another OS (or special purpose boot code), and performs a memory dump. Often times the power cycles quickly enough that useful amounts of data remain in the RAM. This attack can be made more effective if you can get physical access to the RAM and cool it.
As most (if not all) disk encryption programs store an expanded version of the key in memory, there is significant redundancy to recover from the partially lost data.
