I do some programing for applications similar to this in National
Instruments LabWindows CVI (C for virtual instrumentation).  Basically
it allows you to take data (from a number of different sources),
manipulate it and represent it graphically in a Windows 9x or NT
environment.

If you were to take the lines off of the ECU, just like the digifant
tool, you could get data from a variety of sources.  However I do not
see a practical way to use a Palm PDA, at least the 3com line because
the constant updating of the screen would eat the batteries not to
mention the backlight would have to on to reflect off of the
windshield.  The other problem is that the Palm's run Palm OS, you can
write stuff for them, but not with CVI.  The Windows CE PDA's might be
able to run the programs, but I won't get in to that because they suck.

But one could use a laptop, and with the extra space provided you could
log the data in a spreadsheet so that you could review the engine
performance later or even replay the run.  However there is no provision
for speed with out some way of getting the mechanical speedometer
reading into a digital signal, like a digital speedometer sender from an
audi or something.

I put a screen shot of on of the app's I'm currently working on at
< http://www.251.org/temp/mcd.gif >  It's not the best quality, but
you'll get the idea, the original is over 1100 pixels wide, this one
should fit on your screen.

dave