Hi, I'm not expert on image formats but I've been using ImageMagick's mogrify tool to do bulk rotating of images like this:
mogrify -rotate 90 *.jpg
I've noticed that the images appear to be rotated fine when viewed with a Windows browser but to my horror the images I've been rotating don't seem to render as rotated when using an iPad or iPhone. For some reason the iPad/iPhone seems to think the images are still unrotated.
My hunch is that in the specific case of the iPad, its built-in rotation software* overrides anything on the smaller scale.
:: getting dizzy trying to visualize this ::
* This is very unnerving for the first day or so. If you physically turn the iPad, the whole screen image turns too. You can switch off the option if you particularly want to look at things upside-down or sideways-- or if your hands are too unsteady-- but it's the default behavior.