The MacBook Air has never included an optical drive, though an external unit has been available as an option.
And the Retina 'flavour' of the MacBook Pro also lacks a DVD drive.
Snow Leopard was the last version of (Mac) OS X to ship on a DVD - Lion was primarily available as a download from the Mac App Store and later on a USB thumb drive, while Mountain Lion is so far only available as a download.
Furthermore, Apple has always eschewed Blu-ray.
Now an AppleInsider reader has dug around inside the plist file for Mountain Lion's Boot Camp Assistant and found a hint that the next generation of desktop Macs might lack DVD drives.
The evidence (such that it is) is a list of models that allow the installation of Windows 7 from a USB drive.
Known models include the DVD-free MacBook Air and Mac mini (Mid 2011), but there are also codes believed to represent the next generations of the Mac Pro and iMac families.
Some Mac models that do include an optical drive also appear on the list.
It's easy to imagine a DVD-free iMac in the light of the company's recent notebooks, but given the form factor of the Mac Pro there's no obvious reason for Apple to make it impossible to fit an optical drive internally, even if such a device isn't part of any standard configurations.