According to AppleInsider, internal configuration files in Apple's latest operating system, Mountain Lion, suggest that the next editions of the Mac Pro and iMac desktop lines may not include built-in optical drives.
A plist file from Mountain Lion, reproduced below, shows Mac model versions that can support an optical boot disc or a USB flash drive volume capable of installing Windows to a Boot Camp partition.
Apple's Boot Camp Assistant uses the plist to find which Mac models support booting a legacy operating system from a USB flash drive, which is an important advantage for systems without an optical burner; most of the models in the list don't have optical drives.
Two of the models in the list haven't been released yet: the MP60 (the sixth-generation Mac Pro) and IM130 (13th-generation iMac).
Apple has been giving hints for a while that it wishes to move away from optical drives, enabling it to make its PCs slimmer and less fragile and power-hungry. Check out AppleInsider's article for further details of the discovery.