I have noticed that some of my "portrait orientation" pictures on the DMC-G3 are not rotated, while others are. I thought I noticed this occurring as a difference between by two G3 bodies. In fact, it turns out to be caused by the fact that in order to sense the camera orientation during imaging, the Lumix cameras (at least those prior to the DMC-GF5) rely on the sensors for Optical Image Stabilization (OIS) that are in the lens. So, if you're using a lens without OIS, your images won't be correctly oriented.
Here's the quote I found regarding this in a dpreview review of the Lumix 7-14mm f4 lens:
"One design quirk of the Panasonic G series is that the lens's image stabilization sensors are used to detect the camera's orientation, and mark images for rotation accordingly (as opposed to a conventional sensor mounted in the body). Obviously this can't work with lenses which don't have an OIS unit, such as the 7-14mm. This means that Panasonic owners will find all of their portrait-format pictures shot with this lens resolutely displaying the wrong way round."
(Click Here) if you are suffering from GAS (Gear Acquisition Syndrome) as I am, and want to be convinced to buy the 7-14mm lens.
As far as orientation sensing goes, the newly-announced DMC-GF5 reportedly incorporates an orientation sensor into the body, giving you orientation sensing, no matt what lens you are using. This is according to a review on www.cameraguide.com.
(Click Here) to read the DMC-GF5 review in its entirety.
I was relieved to find that #1, I am not going crazy, and #2, there is not some hidden function setting somewhere on one of my G3s that was different from the other. It comes down to the fact that one camera had an OIS lens mounted, while the other did not.