Adobe Collage was removed from the App store temporarily but will be back in a few days. Lightroom isn't available on tablets, the closest product we have is one called Adobe Revel. It is new and different than Lightroom in many ways but still might interest you.
You can learn more about it here:
You can now purchase Adobe Collage from itunes app store at -- http://itunes.apple.com/app/adobe-collage/id517835526