You'll need a C library, a small handful of C libraries, a web-server and the Linux kernel. SSH would be good, possibly git as well depending on the way you deploy. Ignoring any need for a GUI, I think you could get a rather small system.
If you were going to compile it yourself, it's just missing M4, autoconf, automake and OpenSSL. Otherwise you can use it's Appbrowser (ab in micro, the CLI version) to install Ruby in about five minutes.
Quite true. I suppose in my post I really was assuming starting off from brass tacks, cross compiling from a larger system. You are, of course, correct: if the system must be able to bootstrap itself a more expansive base is needed.
I also neglected to mention any sort of shell. Dash, I believe, is POSIX compliant and rather tiny.
