My bad - didn't notice that in your original post.

I think I'd still recommend an iPAQ. I have an older 3630 that's used
for  running the XCSoar flight computer. I like its screen clarity,
brightness and resolution, but am less fond of MS Pocket PC's GUI: some
stuff, e.g. File Explorer, is decidedly non-intuitive and worse, doesn't
work like the manual said it does. iPAQs are easy to buy (both new and
used) and good documentation of their internals is available. If USB
doesn't meet some requirement (e.g. transferring programs or big data
files) you can fit an expansion sleeve and put CF cards, Ethernet
adapters etc into the sleeve's slot.

I've also used a Palm as the control interface for embedded programs on
model aircraft, but only very old ones (mine is an M105). I find the
Palm harder to read (fairly low res monochrome screen) than the iPAQ
though I prefer its GUI and find the "gesture" pad pretty easy to learn
and use. The monochrome display is readable outdoors in bright sunshine,
which is more than you can say of many full colour displays. I don't
think the Palms have the expansion capability of the iPAQ, but I'm quite
prepared to be proven wrong.

As others have said, you'll have to use MIDP rather than the standard
J2SE development kit, but that applies to all PDAs and I guess you know
that already.

HTH