I discussed this in a few threads a while back. I tried to get a damaged X50v motherboard so that I could remove the CPU and trace the circuitry to see if indeed, the built-in USB host is connected to something in the PDA.
Unfortunately, I have not been able to get a bad motherboard (yet).

If you look at the pinouts of the sync connector, there are a few extra pins. I was thinking that these pins 'might' be used for USB host or for a JTAG interface. I found out about a year ago that grounding either pin 5 or 6 will turn on a 3.3v output on pin 7. That is useful for someone wanting to build an rs232 interface.

I also pointed out that there are references in the registry to BT usb devices but I have a copy of the TI BT interface chip and it does not have a BT interface on it.

Also, the older X50v ROM A03 had a USB Host driver in it but it was removed in the A05 ROM and replaced with a new 'preif' driver that was much smaller in size. I never installed the A04 ROM, so I don't know if it was in that one.

Of course, if someone would like to donate a bad X50/X50v/X51v motherboard for me to trace, that would allow me to figure this out once and for all.