Curiosity piqued from the April issue of CostCo Connection it was time to look for more info on digital watermarking. With an iPhone or iPad or similar smartphone or tablet device a person can get info that's hidden from the ordinary view. "Digital Watermarking - New technology links print and online" was the headline. The sidebar article tells how each new edition of the magazine will be using this new type of interaction.
A Google search brings up some likely places to start finding out more.
http://www.scribd.com/doc/132555474/Digital-Water-Marking technology and technique described
http://appfinder.lisisoft.com/ipad-iphone-apps/digital-watermark.html 50 apps so you can watermark your photos and digital works - iPad iPhone Apps
http://www.dpreview.com/forums/thread/3397207 forum discussions on best programs for Mac.