Andrew, if you already have serial attachment for the palm then building the cable is cake. Here is a wiring diagram that a quick drew up:
Supplies you will need:
Thin gauge wire. the individual strands of wire out of a network cable work great.
1x low amperage diode (I used radioshack IN4001)
1x dsub connector (male)
I would also recommend getting a soldering iron (and of course, solder) if you don't already have one. You can make this work without one, but it isn't as clean.
Just assemble the wires as in the diagram. Make sure that when you assemble the dsub connector that the wire clips are inserted far enough into the connector to make contact (I had to jam mine in with a paperclip).
Here, is a link to MMcd v1.8g (I hate email)
(rightclick, saveas)
http://ice.linuxbochs.com/mmcd1.8g.zip