Setting up synce-hal for Bluetooth connections isn't
difficult, it's just not well tested, and may not
turn out to be the 'right' way.

Depending on your distribution, some of the locations
may vary, this example is for Debian.

cp hal-synce-bluetooth /usr/lib/hal
cp synce-bt.ip-down /etc/ppp/ip-down.d/synce-bt
cp synce-bt.ip-up /etc/ppp/ip-up.d/synce-bt
cp synce-bt.peers /etc/ppp/peers/synce-bt

The first three must be set as executable.

Finally you need to run dund with these options.

--listen --activesync --msdun call synce-bt

There is probably a method in your distro's boot
sequence to set this to come up automatically.

Then connect to the PC from the device with the 
ActiveSync over bluetooth option.
