Thanks to this post I finally (nearly one year!!) managed to get my HP Photosmart 4780 printing properly over wireless.
The symptom is that the printing is ridiculously slow, sometimes fails, and sometimes prints a bit of the output.
The solution is to use the HP Photosmart 2700 driver instead of the 4700 driver.
Open the print queue, choose Printer Setup, Driver, Print Using, Select Printer Software, search for 2700 and select the HP Photosmart 2700. Voila! Super fast wireless printing as it was supposed to be.
Update for Lion – be careful; so far I’m back to having problems with either driver. HP recommends using Apple Software Update to install the correct driver. Am testing it now.