You don't have to use a 48MHz crystal.
The USB SIE, however, must run at 48MHz; the USB SIE can receive its clock from either the crystal oscillator, or from the PLL.
So... If, for example, your crystal is 8MHz, then you can PLL its frequency up to 48MHz and run *both* the CPU and USB SIE from the PLL output.
The advantage of using a 48MHz crystal, however, is that you can run the CPU *faster* than 48MHz (say, 64MHz), and still have a 48MHz clock for the USB SIE (directly from the crystal oscillator).
Also see:
http://forums.freescale.com/freescale/board/message?board.id=CFCOMM&thread.id=5744
-- Rich
