Printing wireless from an iPad - it's now a dream that has come true. Although be careful, there are only a handful of printers that are compatible with Apple's nifty AirPrint.
All of the 12 models are from HP, seven of which are compatible out of the box, those seven are:
Photosmart e-AiO D110
Photosmart Wireless e-AiO B110
Photosmart Plus e-AiO B210
Photo Smart Premium e-AiO B310
Photosmart Premium Fax e-AiO C410
HP ENVY e-Aio D410
Photosmart eStation C510
The remaining five require a firmware update to jump to AirPrint compatibility:
LaserJet Pro M1536dnf Multifunction Printer
LaserJet Pro CM1415fn Color Multifunction
LaserJet Pro CM1415fnw Color Multifunction Printer
LaserJet Pro CP1525n Color Printer
LaserJet Pro CP1525nw Color Printer
But, if you're printing from a Mac, there is always the AirPrint Hacktivator program which will make AirPrint work on pretty much any printer...