So I have a similar problem. I have a Palm IIIc with the serial cradle. I can sync under Mandrake 10.0 with a serial cable, but not this cradle. I checked the cradle. First it works perfectly under Windows 2000 for me on the same machine, same physical serial connection. So I know that its not a hardware thing, but software. Also, since I've tried other distros on the same machine and they work, I know where my difficulties lie.
But I've also tried this same configuration on other computers with older versions of Mandrake (9.1) to be exact. The set works. So I'm thinking it is Mandrake 10.0 not liking my computer or something. However I had the same problem with Mandrake 10.1.
ONE time, after removing everything, and adding only necessities along the way that it needed did it work. As soon as I rebooted, I was back to the same situation.
Any hints are appreciated as well.