I just got back from vacation and upgraded my iPad to IOS 4.2 And quickly found out that AirPrint is only supported with a limited set of printers today.
A bit of browsing around, I found AirPrint Hacktivator. It’s based on some investigation done by several people who used old Apple files from pre-releases of 10.6.5, in which AirPrint was supposed to be supported for printing to your Mac’s shared printer. With that feature inexplicably removed from the final version of 10.6.5, the hackers out there found workarounds.
AirPrint Hacktivator makes it even simpler: It simply edits a file for those out there that don’t want to edit Apple files from a Terminal session.
Simply open up Terminal, and edit /usr/share/cups/mime/apple.types with your favorite text editor (vi, nano, etc.) Don’t forget to “sudo”. Add the following lines
# Inspired by AirPrintHacktivator
image/urf urf string(0,UNIRAST<00>)
I did the edits myself, but if you want, AirPrint Hacktivator does it for you. Easy!