iPad portable printer
This is a list of all the iPad compatible printers.
The only one that looks even somewhat portable is the first one.

HP Envy e-All-in-One series (D410a)
HP Photosmart Plus e-AiO (B210a)
HP Photosmart Premium e-AiO (C310a)
HP Photosmart Premium Fax e-AiO (C410a)
HP Photosmart e-AiO (D110)
HP Photosmart eStation (C510)
HP LaserJet P1102w Printer
HP LaserJet P1606dn Printer
HP LaserJet Pro CP1025nw Color Printer
HP LaserJet Pro M1212nf MFP
HP LaserJet Pro M1213nf MFP
HP LaserJet Pro M1214nfh MFP
HP LaserJet Pro M1216nfh MFP
HP LaserJet Pro M1217nfw MFP
HP LaserJet Pro M1536dnf Multifunction Printer
HP LaserJet Pro CM1415fn Color Multifunction Printer
HP LaserJet Pro CM1415fnw Color Multifunction Printer
HP LaserJet Pro CP1525n Color Printer
HP LaserJet Pro CP1525nw Color Printer
HP Officejet 6500A e-AiO
HP Officejet 6500A Plus e-AiO
HP Officejet 7500A Wide Format e-AiO
HP Officejet Pro 8500A e-AiO
HP Officejet Pro 8500A Premium e-AiO
HP Officejet Pro 8500A Plus e-AiO

Edit: That data is from here: http://www.apple.com/ipad/features/airprint.html