On Mac you can get the encryption key from RAM too, though if you turn on some settings, you can force the Mac to attempt a shutdown when it sleeps, clearing the encryption key until you enter your password later. There's still a DMA leak at startup also, but securing the EFI firmware helps keep things locked down. There was an article on HN recently with step by step instructions on scanning memory for things that look like AES keys when using TrueCrypt.

I suspect without a Yubikey or Microsoft TPM, effectively storing the key outside of RAM, there's not much that can be done to fight this. And of course, physical access means you've got the TPM or Yubikey in front of you. And unencrypted data in RAM. So.... Yeah.