When iOS 4.2 makes it to the iPad, iPhone, and iPod touch in November, we'll finally be able to print from those devices with a native Apple capability called AirPrint. In most cases, you'll need to have a printer that has been shared on a Mac or PC, since AirPrint won't have drivers available for the huge variety of printers that exist.
If you're looking for a direct-to-printer solution, though, HP announced that three of its new printers will be ready for the proprietary ePrint technology, which allows driverless printing from iOS devices anywhere as long as they're Internet-connected. These printers also work with AirPrint.
One of the printers was previously discussed on TUAW: the $299 (and up, depending on features) Officejet Pro 8500A. The cleverly-designed HP Envy 100 e-All-in-One (pictured above, link is to product PDF) is the least expensive of the new printers at $249, while the Photosmart eStation tops the base prices of the HP AirPrint-friendly printers at $399. The latter printer isn't that likely to be attractive to iPad owners, since it comes with a detachable 7" Android-based tablet "internet companion" that would be a bit redundant.
Having the ability to print recipes, boarding passes, and iWork documents directly from the iPad to a printer with no intervening computer is going to be awesome.