I was actually asking two quesions:

1. Why use any GPIO for a pullup instead of using a resistor?
You answered this, to keep the gumstix off the bus when booting.

2. Specifically why use FF_RTS instead of some other GPIO?

Sorry for the confusion.
Sean

Dave Hylands wrote:
HI Sean,

  
My company is currently developing a custom gumstix expansion board for the
Hirose 60 pin connector. One feature we are implementing is the USB client
connector. We see that the FFUART RTS and CTS pins are used in all the
current devices from gumstix that have a USB connector. For example:

http://pubs.gumstix.org/boards/HWUART/B00002-R12.USBCLIENT.png

It appears you are using FF_CTS for a plug-in interrupt and FF_RTS for a
pullup on USB_P. Is this correct? Why use RTS for a pullup?
    

I'm not sure if you're asking why use RTS? or why use a pullup?

The pullup bit is what allows the host PC to detect that the gumstix
is "plugged in" an ready to respond to USB commands. So the pullup is
disabled while the gumstix is booting and the cable is plugged in.

You should be able to use any two GPIO lines. Just change the
build_arm_nofpu/linux-2.6.17gum/include/asm-arm/arch-pxa/gumstix.h file.