The thread on a car monitor with a speech synthesizer (http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1243362861) got me thinking about using iPaqs and Palms in conjunction with Arduinos.
For example, you can buy a working Palm III for not much more than the cost of a bare graphic LCD. There's an app available (http://palmorb.sourceforge.net/) that makes it act like a Matrix Orbital serial character LCD. But why not make it act like a graphic LCD? You could get around the slow speed of the serial interface by implementing the functions of the GLCD library (http://www.arduino.cc/playground/Code/GLCDks0108) on the Palm, and just sending the function parameters, instead of hundreds of bytes of pixel data. You could also use the Palm touchscreen as an input device for the Arduino.
It costs more to get an iPaq, but you get a much nicer screen (again, at a cost similar to that of a bare display module). And quite a bit more CPU power, so you could do both a display and a speech synthesizer on it. And even use it as an Ethernet or WiFi bridge with the right plug-in card. Or write apps that run on the iPaq and use the Arduino for I/O. I doubt an iPaq could support a full-blown Processing environment, but it's probably feasible to do some sort of standard GUI application that would use (a subset of) the same protocol.
My inclination (because of my strong anti-MS bias, and my extreme cheapness) would be to design tools that would work with older iPaqs that had been reflashed with Linux. It's a more PC-ish environment, with networking and multi-tasking well-supported, that's just plain better for building embedded applications (and doesn't require a hard reset and restore every few weeks like older versions of the MS software do :P).
Has anyone else thought about doing this sort of interfacing? What are your thoughts on architecting a PDA-Arduino system?