Check out the new ink less printer. Yes ink less! No Cartridges! The Dell Wasabi PZ310 uses ZINK (Zero Ink) Technology for printing without ink. The special ZINK Paper has crystals that when heated produce all the colors of the rainbow. For a more geeky explanation click here: How ZINK Works.
The Wasabi printer is tiny and fits in your back pocket. Of course this means that the printer will only print on 2x3" paper. That's about the size of a business card. The printer can hold 12 pages and the ZINK Paper has a sticky back so you can print 'Kick Me', peel the back and stick the print out on random stranger's backs as they pass by.
The Wasabi uses Bluetooth (FTP, BIP and OPP) to communicate. If you have the feature retarded iPhone with its crippled Bluetooth then sorry no mobile printing for you. Buy the Android instead.
Lastly, if you're not a Dell fan you could go with Polaroid's Pogo Instant Mobile Printer or Instant Digital Camera or Fuji Instax 200 Instant Digital Camera.