im sure, i havent dropped mine yet knock on wood, but i would drop my inc all the time and as soon as it hit the ground the thing exploded (battery, cover) so yeah im gonna say thats normal lol.
Use to happen to me with my Droid all the time - the back cover and battery would go flying. It didn't bother me at all - these things happen when you drop your phone, it's better than stuff cracking or shattering into tiny pieces, lol.