Preview at WWDC likely in June, followed by September launch.
'MacBook Air' Design Considerations
For Apple to launch an extremely slim MacBook laptop, however, there are a few design considerations that should be recognized. With the reported removal of the optical drive from the body of the machine, it seems clear that Apple is interested in slimming down the laptop as much as possible. In creating a super-slim laptop, there are a few other components that become size-limiting.
While there has been much talk about the move to Solid State (Flash) based drives, these drives carry a significant premium over traditional hard drives. Apple uses 2.5" SATA hard drives in their current MacBooks. Regardless of type of drive used, moving to a 1.8" form factor would be necessary to slim down the laptop further. Apple has a significant amount of experience with the use of these 1.8" drives which are used in the hard drive based iPods. Of note, Samsung's recently announced 1.8" 160GB drive has started shipping this past week for only $199.
Another size constraint would be the Ethernet jack on the laptop itself. We learned that Apple had been exploring this issue through a published patent application that explored the possibility of collapsable external ports. Other manufacturers of thin laptops have used ethernet-usb dongle to avoid housing the ethernet jack on the laptop itself. Similarly, Apple would have to include an attachable ethernet adapter, if their collapsable connectors never graduated from the research lab.
Apple is rumored to be releasing a new slim MacBook at Macworld San Francisco. User conferences begin on January 14th, but the keynote kicks off on January 15th at 9am. We'll have live coverage at MacRumorsLive.com and extended floor coverage at MacRumors.com/mwsf.