I too could never get visor and /dev/ttyUSB* to work with my 700p, so I
switched to libusb, which is faster.
Try this, it works for me with a 700p on Debian etch:
* find /lib/modules -name visor.ko -print
* mv <that file> /root/ [if there's more than one, give them different names]
* rmmod visor
* <Remove pilot-applet from panel, if it's running>
* apt-get update && apt-get install libusb libusb-dev
* cp /usr/share/pilot-link/udev/60-libpisock.rules /etc/udev/rules.d/
* /etc/init.d/udev restart
* <Plug in Treo>
* <Hit hotsync>
* pilot-xfer -p usb: -l