A little Googling resolved a bizarre glitch with VirtualBox and processor usage.

My desktop boasts a 2.66 GHz Core 2 Duo processor with 6 GB of RAM.  VirtualBox hosting my little CentOS server and development environment would peg one CPU core regardless of load.

It seems ridiculous to have an idle VM consume 50% of my CPU cycles.  According to this page, the solution is two idle VMs.

So, I configured a second virtual machine with 4 MB of RAM, no operating system, and no fixed disk. Sure enough, my host system CPU usage dropped to around 2%. Strange.