I use the Audiovox RTM-8000 with no problems, I prefer to have it use the Z battery, since it reduces the clutter.
I have written a how-to document, posted here in the communications forum, and uploaded the serial_cs.o file along with the scripts and instructions.
Note that it works with voice as well, you just need to type the dialing string to dial. I know some who do that with their Z\'s, I haven\'t since I only pay for data.
But I do agree that manufacturers don\'t care about the Linux PDA market, it\'s just too tiny for them.