Hrm... are you testing in the emulator or on your real phone? What version of Android? There seemed to be a problem with Samsung in version 2.x that had this problem. Another thing to try would be to use the android adb tools to connect to the device and see if the file already exists - perhaps another app had created one with the same name?
Anderson gave himself the promotion. So I gave myself this tiny ad:
The Low Tech Laboratory Movie Kickstarter is LIVE NOW!