I had the same problem and just figured out the reason: starting with iOS 4.0 when the camera takes a photo it does not rotate it before saving, it simply sets a rotation flag in the EXIF data of the JPEG.
If you save a UIImage as a JPEG, it will set the rotation flag.
PNGs do not support a rotation flag, so if you save a UIImage as a PNG, it will be rotated incorrectly and not have a flag set to fix it. So if you want PNGs you must rotate them yourself.
I would call this a bug in the PNG saving function but that’s just an opinion (they should at least warn you about this).