Yes, very easy. In a mac you just plug in a Firewire/thunderbolt device[1]. More extreme measures involve freezing the RAM. Both require physical access to the machine, but a bit more scary that plugging your laptop into a public display/TV gives an attacker control of your computer and passwords.

Full disk encryption TrueCrypt/BitLocker/FileVault can act as countermeasures[2] and modern versions of OSX don't allow DMA from the login screen anymore.

[1] http://www.breaknenter.org/2012/02/adventures-with-daisy-in-...

[2] http://www.researchgate.net/publication/49277520_Cold_Boot_M...