Open iTunes when iPhone is connected on a Mac
Go to System Preferences → Accounts, and view your Login Items. One of them should be
iTunesHelper.

If iTunesHelper is missing from Login Items, here's how to add it:

iTunesHelper is located inside the iTunes.app bundle, at

/Applications/iTunes.app/Contents/Resources/iTunesHelper.app/



Navigate into the iTunes.app package; in Finder, Right-click iTunes.app and select Show Packaged Content to get inside it. (Or use Terminal to navigate and then open Finder with open .)
Locate iTunesHelper.app, and drag it to the Login Items pane. 
Logout or even restart may be required for changes to take effect. 


Whew, that was weird. I'm glad I got it added back too. :-)

(Alternatively you could reinstall iTunes, but the above is much quicker and easier.)

These forum threads were helpful: 


How to prevent iTunes from opening when iPhone connected
iTunesHelper is gone in login items...