I did a little more playing with it, and it's just the weirdest thing.

Photos sent from my iPhone through the native Photo/Email app aren't oriented correctly on DW when I view them from a computer (other people see them mis-oriented as well). Viewed on my iPhone or iPad, they are oriented correctly.

If I process the image through something other than the native Photo app, it's oriented correctly on all devices.

So maybe there's some metadata that is associated with the photo that iDevices can read? Not really sure. Maybe my iPhone is just perverse.