You can retrieve the key if the computer is turned on by dumping the memory from a Firewire or Thunderbolt port. If these ports are disabled you could use the Cold Boot Attack. This is the same with all computers, it's just that Apple continue to make them with Thunderbolt.
Apple has made some progress on protecting from DMA attacks -- when the screen locker is enabled, you can't do it. The kernel uses Intel VT-d for this; I'm not sure if Linux or Windows do the same yet.
I knocked Apple a bit for security issues 2006-2009, but they've made a serious effort to fix things starting sometime in 2010 or 2011. I mean, iOS and the iPhone platform is probably one of the most secure mobile OSes now (RIM edges it out, but RIM sucks). OS X has added other security features as well, starting with 10.6.
There may be other ways...
'Unlocking FileVault': http://www.youtube.com/watch?v=doGzuOYCNJE