Back in March, in preparation for our recent family roadtrip vacation, I purchased the Polaroid PoGo Instant Mobile Printer. The tiny PoGo prints 2 x 3-inch borderless color images in under a minute onto ZINK (zero ink) paper.
It has a rechargeable battery (the power brick is bigger than the printer itself!) and can connect to digital cameras via USB (using PictBridge) or even cell phones via Bluetooth (except, unfortunately, for the iPhone). A 30-pack of ZINK sheets goes for about $8.70 on Amazon so that works out to $0.29 per print.
The idea was my daughter could use it in the car while we were on long drives between vacation stops to print photos from her camera and put them right into her scrapbook. It was a hit! While I was driving she could sit in the back seat and review the photos on her Kodak EasyShare C613 or my Canon PowerShot SD600 and print out her favorites. Then she would peel off the backings and stick them right onto her scrapbook pages. She probably printed about 25 photos on a single charge.
I hope at some point Apple introduces Bluetooth printing into iOS 4 because being able to send photos to the PoGo right from my iPhone without a cable would be pretty cool.