It turned out to be really easy to do Bluetooth HotSync from my Treo 650 to my Fedora Core Linux box.
On the Linux box:
service bluetooth start service dund start
Create the /etc/ppp/peers/dun file:
115200
192.168.2.15:192.168.2.16
local
ms-dns 192.168.1.14
noauth
debug
The first IP address is the local machine and the second is the Treo.
On the Treo 650, open the Prefs, Connection. Create a New connection, with PC, Bluetooth and then select the computer. Next, go to Network prefs. Create a New (from menu) service. Select the Connection you just made. Set your username and password (although I suspect they aren’t needed).
Then, open HotSync. Set the Modem Sync Prefs to Network, the LANSync Prefs to LANSync, put the PC IP address from above in Primary PC Setup box. Finally, select Modem and select your Bluetooth network connection below. Start the HotSync.
On the Linux box, you can run any pilot-xfer command using net:any as the port.
pilot-xfer -p net:any -l
For some reason, HotSync over the Bluetooth serial connection does not work with Linux.