Malc,

ERMEGM is not using a 18F4550 in the board. He is using a 16F874A.
It's max rate is 20MHz using HS mode.

The 18F4550 is a bit of a different animal as far as I can tell looking at the DS.

It can take a 20MHz input in HSPLL mode that then gets divided by 5 to drive a 4MHz input into the PLL to get a 96MHz PLL clock. You can then select to divided down to 48MHz for the MCU clock. This allows the USB and MCU to run from a common source but at different rates.