Apple would still need to adopt USB 3. Lightpeak is just a common cable connector and setup for sending data around, it however doesn't specify a protocol. You would still need to send USB, or DisplayPort, or whatever signals down the cable to the devices. One possible use for this is a computer that only has a single power plug and lightpeak plug. That lightpeak connection could run into a monitor, that then the monitor provides USB, eSATA and audio plugs for devices, and a downstream lightpeak port for extra monitors or other devices.

Apple hasn't adopted USB 3 yet because Intel and NVidia haven't. Apple only uses Intel chipsets in their higher end systems with Core i series processors, and NVidia ones in their Core 2 Duo based systems.

Any motherboard that does support USB 3 today is doing so using a 3rd party USB controller currently. Intel is supposed to have support added to their core chipsets in 2011.