I need to build a Linux image for VirtualBox that includes just enough software to run a Ruby on Rails stack -- basically the kernel, the networking stuff, Ruby 1.9.2, Apache2, mod_rails, and git. Any thoughts on the simplest way to make that happen?
A basic server install of Ubuntu is pretty bare bones. If you want to go even smaller, you could try Puppy Linux http://puppylinux.org/main/Overview%20and%20Getting%20Started.htm
I typically use server installs of Ubuntu for my apache and database VMs.