Shameless startup promotion here: PrivateCore (www.privatecore.com) is working on protecting against DMA and other memory extraction attacks, like non-volatile memory.
Our model is that no hardware is trusted except the CPU and TPM. Plaintext data never leaves the CPU. All of main memory is encrypted, so it's beyond just protecting crypto keys in registers like TRESOR.
Check out the attack device we implemented and how we defend against it: http://www.privatecore.com/dma-attack-video.html