It's not driverless printing. The drivers are simply loaded into the device already. Love the shell game that Steve attempts to play. Perhaps he fools idiots with it, not sure.
It depends on your definition of driverless. It's either driverless or single driver printing. And it works because the printers (or the computers they're attached to) have gotten smarter.
AirPrint uses IPP (Internet Printing Protocol) and Bonjour to find and communicate with printers or print servers. The reason it's called driverless is that there's no printer-specific code required. AirPrint sends PDFs to the printer and the printer is responsible for transforming the PDFs into print pages.
Printers got cheap years ago because they moved all of the processing to the computer that was attached to them. Each printer accepted complex (and printer-specific) codes that the computer had to create. Cheap embedded processors have gotten very powerful so even cheap printers are able to print from the Internet, from memory cards, etc. That means you can send complete print-ready documents to those printers and not have to know anything about how the printer works internally.
The 10.6.5 AirPrint support (that was pulled) worked by providing an OS/X layer that accepted PDFs. OS/X would accept a PDF on behalf of a printer and use the normal OS/X print drivers to print it on the printer. It works just fine. The rumor is it was removed from 10.6.5 not due to technical issues but due to patent issues.
The AirPrint mechanism is generic, not Apple-specific. Some newer HP printers work with AirPrint because they accept PDF documents w/o requiring a driver. HP did this to provide support for printing from the Internet or via email. Linux operating systems (which use the same printing infrastructure as OS/X) can also be used as AirPrint servers, just like the 10.6.5 betas did.
Apple (and HP) picked this approach because it's "easy" to support and has no appreciable downside. There is public domain software for rendering PDFs (and PostScript) and IPP is a public standard for printer communication.
As often happens when Apple picks up a new technology standard there's a lag between implementation and widespread support. It'll probably take a year or two before other printer vendors provide equal support to HP.