Android's Back button (whether in hardware or software) is braindead. I can understand the technical reason for its behavior. It's supposed to take you to the previous Intent...
...unless the current app has overridden the Back button.
Here's an example: Yesterday I attended BLDR.UX11. When I got into Boulder, I pulled over and pulled up my email with the ticket confirmation in it (using my Nexus S phone, not an Android phone with crapware on it). From there, I opened the conference web site. I then found an address for the venue and clicked on it, opting to open it in Maps. Once there, I had Maps give me directions to the venue, and then hit the Navigate button for turn-by-turn navigation. When I got to the Boulder Events Center, I grabbed my phone and hit back, which took me to the Maps app (from Navigation). I hit back again and it went to the BLDR.UX11 page in the browser. I hit back again. WTF? Now I'm looking at an article I read on my phone while waiting in the doctor's office a week ago. At this point, I repeatedly mashed the back button out of frustration until I was at the Home Screen (I realize I can use the Home button-I blame muscle memory).
While at BLDR.UX11, I met lots of folks who feel that having a single unified experience across mobile devices is a myth, and almost all of them cited the Back button as the reason. Some people go as far as to create their own back button in the app, which just makes the matter worse: now you have TWO back buttons.