I found this great tip over at Mac OSX Hints for iphone users.
I have a laptop and use an external hard drive for my iTunes library. This system works well for me, except sometimes I found myself frustrated that I couldn’t update my contacts and calendars on my iPhone while I was on the go. When I’m away from the external hard drive, iTunes complains.
I recently discovered that if my iPhone is charging over USB, using the Sync system menu (chasing arrows in the upper right corner of the screen) on 10.5 syncs my iPhone to iCal and Address Book. This is great! Now I don’t have to launch iTunes unless I want to sync media.