So to set up a new machine, I simply copy my .emacs over and start Emacs. Emacs will then fetch and install all my plugins automatically. Quite handy!
(dolist (source '(("technomancy" . "http://repo.technomancy.us/emacs/)
("elpa" . "http://tromey.com/elpa/)))
(add-to-list 'package-archives source t))
(defvar my-packages '(starter-kit zenburn-theme auctex color-theme-solarized csharp-mode ecb_snap find-file-in-project flymake-css flymake-php idle-highlight ido-ubiquitous ipython python-mode magit markdown-mode paredit pastels-on-dark-theme php-mode rainbow-mode smex solarized-theme starter-kit-js zenburn-theme)
"A list of packages to ensure are installed at launch.")
(dolist (p my-packages)
(when (not (package-installed-p p))
Then everytime you installed a package it would get written into the list and everytime you moved your init it would all happen automatically.
That would be cool. And not hard to do either. You could just keep a customization variable of the "base packages" or something and have it saved by the standard custom stuff.