I never knew it was there... -.-
Click the magnifying glass next to the search box, and it brings you here:
https://bitcointalk.org/index.php?action=search;advancedIt brings a little sanity to the search process, even though google => site:bitcointalk.org works best.
Posting for all those that missed the button too. It should ideally be button shaped, right now it looks like a graphic that does nothing. Or, it could be changed to text - "Advanced Search".