The reason I put this up here on Hacker News is that I did not know that they were going to be coming out with the ability to print from the iPad.
Requires hacking with currently released version of Mac OS X because there's some controversy about when/if AirPrint-through-a-Mac-or-PC will be coming. Some say it's cancelled, but that sounds kinda crazy to me: why even ship the feature on iOS if it's only going to be able to print to a small portion of HP's line of printers?
It's because they don't want to start having user-installable device drivers on iOS (a laudable goal); I assume HP ePrint printers work because they speak something very PDF-like natively, though that's purely conjecture on my part.