Well, i've been looking in to this and found an easy couple of installs to do it (you can modify your info.plist files but this way is easy, and 'reversible')
You'll need SIMBL installed first
http://www.macupdate.com/info.php/id/18351
then go and get megazoomer
http://ianhenderson.org/megazoomer.html
Just copy the megazoomer 'bundle' into the SIMBL plugins folder and you're done (my plugins folder was under root drive at \Library\Application Support\SIMBL\Plugins since I installed it for all users)
Then you can use a hotkey (Command-Enter is initial default) to 'megazoom' - excellent, and works for Safari and
any Cocoa based apps (but sadly not for older Carbon ones... think i got that the right way round?). Works great for me for Pathfinder too... but not MSoft Office 2008 or Opera browser sadly (or Firefox).... almost making me reconsider moving over to Safari.
ps before anyone chimes in with the 'why on earth would you want to expand a window to fill the screen' etc etc blather - i'm using the 13" Mbook Pro, and previously a Sony TX with an 11.1" screen... so have always tried to clear way unnecessary clutter to give maximum screen space to stuff that's useful and needed,
especially vertical space, even more useful on today's widescreen laptops.
This simple keystroke removes the title and menu bar (yet you can mouse up to the top of screen to reveal the menu bar again, kind of 'auto hide' stylee) and repeat the keyboard shortcut to go back to normal.
Most excellent! (and free).
Coops