If using a Palm or even a CE type machine, I'd rather go with an ethernet connection than serial. More expensive for sure, but a lot easier to customise an interface for
Palm's don't support ethernet, but you can still customize the interface.

Well, I know that there are thousands of applications that can run on a $99 Palm. Using a Palm with PalmEmpeg gives you the advantage of two-way communication.

This post talks about the possibility of making PalmEmpeg work via tcp/ip.

The Palm does IrDA but can also do raw IR. For transferring data between the Palm and the empeg, see Hugo's post. I read another thread where someone got TCP/IP working over IR between a Palm and empeg, but I can't find it now.

With a Palm, you could also display your gps nav maps on a screen with many more pixels than the empeg's front panel.

All of that to say that there's so much more you can do both interface wise, application wise and communication wise with a Palm... for one quarter the price.