The thing about the Android back button is it's highly modal and it doesn't communicate the mode. For example, if I'm in a web page, it might go back to the previous web page. If there's no previous, it goes back to the previous app. There's no visual change to indicate the different behavior.
You say that like its a bad thing.
The back button isn't modal, it's contextual. It does exactly what you need it to in context, if you want to go back to a search page, you press back. If you want to go back to the previous application, you press back. If the keyboard is up, pressing back makes it disappear.
With the Iphone you cant switch easily between applications. If I want to go from the phone to SMS, I have to go back to the homescreen and open SMS, if I want to go back to the phone I have to go back to the home screen and open phone.
Mind you, I very much like the Android-style back button, but it took me a long time to learn how it would behave.
It really isn't that hard, it took me about 20 seconds to learn all it's functionality. It took my technologically illiterate dad all of 10 minutes to figure out what back did on his Galaxy Nexus whilst my mum (the parent that actually uses a computer) still hasn't figured out her Iphone.