If this application is to be designed for iOS then placing the button on the right is actually a violation of Apple's Human Interface Guidelines for the Navigation Bar (see the Guidelines section):
A back button should appear to the left of the title, and it should be labeled with the previous level’s title.
If this application is being designed for Android, Windows Phone, and (I believe) BlackBerry a back button is not needed and would be redundant as there is a hardware button on the phone designed for exactly this action. If you're building for WebOS I believe there is a gesture used for back and thus no additional back button is needed.
Placing your back button on the right can cause confusion as users may not consider the shape and content first, but rather the placement and assume it means "next/continue/forward".
Placing the button on the left puts it at the front of left-to-right readers' visual scanning pattern and thus it's easier to find. If you're designing for right-to-left reader's exclusively, then you might be able to justify a right-hand placement, but you'd need to flip your icon around so the arrow points to the right as well.
See also this site for further examples on why placing the back button on the right may be a bad idea.