A recent post from David Hyatt on the www-style-list answers my question from April 15th: Will we see CSS variables next?. The short answer is yes!
WebKit now has an experimental implementation of CSS variables:
You can test this feature using a WebKit nightly
Thanks to David Hyatt and Daniel Glazman for bringing something to CSS that many of us have wanted for nearly a decade! For additional information, go read the discussion thread.