There are already such apps I believe. I have printed wirelessly to my Epson using "Epson iPrint" app, but its only for photos. I dont think a 3rd party app will be allowed to print from any safari page, email, etc directly from the source though.
I dont see any reason why Apple could not have done this right from the start.--execpt one maybe: exclusivity deal with HP. I mean really, there is no other excuse. Apple programers cant be that lame that they couldnt have got this working correctly.