Since it looks like you are feeding two (or maybe 3?) bathrooms on that 20A circuit, then I agree that it looks like you cannot feed the fan off of your GFCI circuit. There is an exception to 210.11 (it is on the next page from where you are looking if you are using NEC 2008) that allows you to run the other bathroom fixtures (lights, fan, etc.) off of the 20A circuit IF you are only feeding just that one bathroom with that circuit.
Could you move the fan so that it isn't directly above the shower and avoid the GFCI? Another option would be to take the lighting circuit, run to a box with a GFCI (could be a dead-front style (no outlets)) and then connect to your fan.
If you haven't bought/installed the fan yet, you should also look at the Panaasonic fans. They are quiet, well built, and move a good about of air. I highly recommend them.