That being said, even with volume encryption, the key is in memory somewhere. RAM isn't wiped as quickly as you might think, and it's apparently possible to extract keys from memory after the machine has been powered down. So be warned, and be paranoid!
There was an article I read recently about a patch for the Linux kernel to store the key in CPU registers instead of memory as well...
Take one of those, and a Google-style Lead-Acid internal UPS, and you'd have an incredibly hard time getting access to keys held solely in memory.
For the extra paranoid, accelerometers or a simple mercury vibration/tilt switch would make it even harder, at the risk of losing/hanging your server every time someone worked on the rack, or drove a heavy pallet cart nearby.
I recall seeing somewhere a device designed for law enforcement, which was essentially a cart-style UPS with probes designed to be clamped onto, and penetrate, the power cable, allowing seizures without any power-loss, which I assume is for this eventuality.