Contrary to conventional wisdom, “volatile” semiconductor memory does not entirely lose its contents when power is removed. Both static (SRAM) and dynamic (DRAM) memory retains some information on the data stored in it while power was still applied and they still hold values for a long intervals without power or refresh. This is a known  problem for a long long time. However, no one has ever tried (or published) any practical attack on this problem like what Princeton University researchers did.
This DRAM threat goes beyond disk encryption. Any kind of sensitive data such as password, encryption key, credit card information,… in you RAM could be stolen in just a few minutes. Due to the nature of this problem, it’s hard for software based hard disk encryption solution to protect against this attack. Software based solution would be able to try to encrypt/clear the disk key whenever PC goes into inactive state (i.e screen saver, standby, hibernate) but it’s not really practical and/or applicable in some cases. The white paper  also offers interesting algorithms & methods to find crypto keys in memory images.
If you’re really care about your information, you should better to change your behavior to unmount encrypted disk and/or power-off your machine (for a while to give the memory enough time to decay) whenever you’re away from your computer if you’re using software based disk encryption and/or to use a hardware based disk encryption solution. FYI, Seagate also has a hardware based hard disk encryption solution ready to use.