Shown here is a picture of the SiTime SI8002AC package with its outer protective casing removed.

An ASIC - which converts the raw signals of the oscillator - is stacked on top of the oscillator itself, and the two are wire-bonded together for signal transfer. This entire unit is completely sealed inside the plastic outer package.

The second picture is an X-Ray of a stacked die configuration of a Bosch BMA 220. The wire bonds connect the dies both together and to the ball grid array.

Stacking dies allows chip manufacturers to pack more functionality on the same footprint. This is especially important in mobile devices such as the iPhone 4, where board space is minimal.