Panasonic's G1, G2 and G3 micro four-thirds cameras are missing a dedicated orientation sensor. With a lens featuring image stabilisation the camera makes use of its sensor. Otherwise (e.g. with the Panasonic Lumix 20mm/1.7) the Exif image orientation tags are not correct for portrait format images.
Here is how to write correct tags (compatible with the camera's preview function) with exiv2:
Camera rotated anti-clockwise (shutter button on top):
exiv2 -k -M"set Exif.Image.Orientation 8" -M"set Exif.Panasonic.Rotation 8" -M"set Exif.Thumbnail.Orientation 8" P1020800.JPG
Camera rotated clockwise (shutter button at bottom):
exiv2 -k -M"set Exif.Image.Orientation 6" -M"set Exif.Panasonic.Rotation 6" -M"set Exif.Thumbnail.Orientation 6" P1020800.JPG