They are listed on the apple website
http://www.apple.com/ipad/features/airprint.html
Works with AirPrint-enabled printers.
AirPrint works seamlessly with a new generation of wireless printers from HP.
AirPrint-enabled printers
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 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