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, you like to be in control of software updates, and you care a lot about homebrew packages being up-to-date, you are probably better off adding a line to the bottom of .bashrc
(or the rc file of your favorite shell) asking:
brew outdated
This will confront you, in every new terminal, with the outdated packages.
When your annoyance reaches the limit (you feel ready to update), the brewup
script that @bmike suggests mightyou'll be your next friend ;-)happy to do the necessary.