For Linux there is TRESOR (http://www1.informatik.uni-erlangen.de/tresor). It uses CPU registers, which can prevent this sort of attack. However, you're limited to one encrypted drive with this method.
You can do a lot with that space. My company PrivateCore runs an entire Linux/KVM stack within the L3 cache, then fully encrypts main memory. Someone physically acquiring memory would only obtain plaintext.
Note that a software compromise could still read memory. That's one reason why it's necessary to fully attest a system before provisioning it with any keys or passwords.
Here's a CanSecWest talk with more details: http://cansecwest.com/slides/2013/PrivateCore%20CSW%202013.p...
Several physical attack vectors can modify the contents of memory, thus compromise the software stack and divulge keys kept in registers or cache.
Our approach at PrivateCore is to fully encrypt main memory with an authenticated cipher mode and keep the software stack pinned in cache. An attacker able to physically modify memory can only conduct a DoS attack by inserting junk data.