You might want to ask yourself: "Do I really want to auto-update applications, which the applications that I build myself depend upon?"
Brew team turned down this feature request to facilitate early updates.
My interpretation of their reasoning: "The risk of malicious code slipping through the opensource auditing process weighs more than the incidental need for a speedy update."
If this is your line of reasoning and you really care very much about being up-to-date, you're probably better off adding a line to .bashrc (or the rc file of your favorite shell) asking:
brew outdated
It will confront you with outdated packages in every terminal you open. If you get annoyed enough (feel ready to update), the brewup script bmike suggests might be your next friend ;-).