Hi. PrivateCore has implemented encrypted RAM as part of a secure hypervisor. Our product is currently in a private beta, but you can check out our website at: http://www.privatecore.com.
OpenBSD does this by default. It also now directly boots cryptodisks eliminating the need to create a /boot partition and carry it around if you're concerned about evil maid attacks, though I would imagine a camera or keyboard hardware keyloggers would defeat that pretty easily
If you stream the data in from disk (e.g. read 1kB, encrypt 1kB, read 1kB, encrypt 1kB) then a cold boot attack only recovers one small chunk of data. Whereas if your keys are in RAM then a cold boot attack can recover the keys which can then be used to decrypt your entire hard drive.