In the iPhone Google Maps application, it's pretty obvious how to zoom into maps. You double tap the screen. The map readjusts, zooming further in. So how do you zoom out? Sure you can pinch your way back but there's a far easier way. Use a single multi-touch tap instead. That is, tap with two finger at once.
Separate your fingers so it's clear that you're making two contact points and...tap. Google Maps obediently zooms back out, one zoom stage for each multi-touch tap. Give it a try. It's a great tool to bring into your Google Maps vocabulary.