Is "suspend-to-disk" memory dump encrypted when using FileVault2? I am conserned about cold-boot attacks on my full disc encrypted (filevault2) laptop. 

Therefore I am wondering if the memory image dump to /var/vm/sleepimage is encrypted before it is saved to disk?

I notice that the suspended drive asks me for my password before unlocking so it appears this is the case, but could anyone confirm this?
Everything written to the volume under Filevault 2 is encrypted live and in-stream by the kernel / device drivers.

As long as there isn't a bug where data leaks to outside the core storage wrapper, you don't need to worry about someone analyzing the contents of your RAM by snooping the locked drive.