This guy has an interesting workaround for the general lack of the industry making a truly portable AirPrint printer.
http://genuinely-original.blogspot.com/2012/01/getting-ipad-to-talk-to-h470.html
And the techy details of how to connect the devices....
http://genuinely-original.blogspot.com/2011/09/printing-from-ipad-iphone-and-ipod.html
He's putting the portable printer into Ad-Hoc WiFi mode (not all printers, and even fewer portable printers, will do this)...
Then connecting the iPad to that Ad-Hoc network (which means you won't get e-mail or have access to Dropbox, etc once connected to the Printer's WiFi network...
Then using HP's printing App (free in the App store, and works on more than just AirPrint printers, but has the limitation that if a document reader App does not have an "Open this document in... <another App> button, there's no way to get the document to HPs printing App unless it's one of the native iOS items, like Photos, which the HP App has direct access to the Photo albums...
Some have pointed out that ipad3 has hotspot capability and the printer may work just connected to the hotspot WiFi network the iPad generates.
Other options include bigger non-portable but small(ish) AirPrint printers and a power inverter for automotive use.
Even HPs latest mobile printer model doesn't support AirPrint but probably works with their App with above noted limitations.