Apple Unable to bundle Bluetooth Accessories
MacSlash posts a report which explains why Apple Wireless Mouse and Keyboard are not presently a build-to-order option at the Apple Online Store.
Based on comments from an Apple representative, Bluetooth Devices need to be configured before their use. If they were to ship new Macs with only Wireless accessories, new users would have no functional keyboard and mouse in order to setup their systems.
Reportedly they are working on work-arounds.