I discovered Helicon Remote a few days ago. It allows you to hook your Android phone to a Canon or Nikon DSLR via the camera’s USB port and control many aspects of photo taking. It even has live preview on the phone screen which is a nice bonus.
The app itself is free to download but is limited in that it won’t allow you to shoot RAW images. It also doesn’t restore the camera settings when you shut it down so you may need to fix those after using it. If you register it by handing over $38 (discounted price, it’s normally $48!) you’ll be able to make RAW photos.
I tried the Helicon app with my SGSII, an OTG USB cable and my Canon 40D. Images are stored on your phone just in case you’re wondering where they’ve disappeared to after disconnecting everything!