Ok maybe not as a post format...that's ok. just an idea. I just thought given the popularity of posting mobilely that it might be interesting to have a way to present mobile posts differently just like we can with the other formats. I thought post formats as a starting point for this idea because the device origin would be able to be archived too.
Anyway it was just to see if there was a way to give mobile post some different indicator of where the post was published from, (ipad, droid, etc.) other than that signature that currently is available.
As Pat said above, maybe a setting on the mobile apps...to set device origin, just like the Geotagging of posts.