From HP's site;
"What is the difference between Apple’s AirPrint and HP ePrint?
HP ePrint is a free service from HP that allows you to print from any mobile devices to your Web connected printer over the Internet. It is as simple as sending an email to the email address assigned to your ePrint-enabled printer or multifunction devices."
I just bought an HP Photosmart 7510 All-in-one during their Memorial Day sell. But what they say above ain't necessarily or exactly so. Some Wireless HP printers can operate in the same (or similar) manner as Airprinters. Other wireless (and to a lesser extend some wired) printers can operate with iPad using appropriate apps (see eprint & weprint for example) and software running on your PC. Those eprinters from HP (check their site for exact models & remember that not all wireless HP printers are eprinters) will work with the apps that can use Airprinters, e.g. Email, Safari. And there is a way to 'email' webpages to the ptinter once you have an HP eprintcenter account (free) but there are restrictions - e.g. a size restriction (not sure exactly what size is max), you MUST have something in the Subject line, You can not have a 'Signature' in your email or in a forwarded email. Also Windows 7 (64 bit) has a problem with scanning if the printer is an all-in-one. In addition I had a lot of trouble getting an account to their eprintcenter. I eventually had to resort to their tech. support and they had to manually set it up.
Currently HP is the only printer Mfg., other than Apple, that makes a printer compatible with Apple's Air Printer standards.