For what it is worth..
I have done a fairly exhaustive hunt myself ..
not just for linux handheld devices....
but virtually any umpc or handheld devices for that matter...
that have a physical mic in and audio out set of ports..
NOT a unified single mic/headphone port. (just soooo much of a pita...)
I am speaking primarily of utilizing real time effects processing (properly...we have a running kind of sloppy mess that kinda works..)
Shy of the raspberry pi utilizing the wolfson cape/card (pretty wicked actually)
Or a very old (rather out of stock ) audio cape with mic in audio out for the beaglebone black (no availability of that cape makes it rather useless)
there is the sony vaio vgn-ux series...which has a dedicated mic in and audio out set of ports...woohoo!
.which can run linux distros...
and has been mentioned here in this thread as an option...
BUT...I have read soooo much on the ux line and utilizing linux distros...
and though it is possible to run it on the ux models..all of the models have issues...primarily...what comes up as the the cripplers are
wifi issues...
touchscreen calibration...
, touchscreen accuracy...
(these last two bug me since using full linux fx processor progs..
hell...pretty much any progs..
on a screen so small and tight means touchscreen accuracy and compliance
(meaning where you try to precisely touch will actually BE where you touch) is freakin' dodgy...
to the point many people opt for keyboards and mice instead of fighting with the touchscreen...
soooo...
adding a ton more peripherals not because you CAN ...
but because you MUST...ticks me off..)
In all it ticks me off since the ux line looks/looked like the best option

I mentioned in the guitar effects thread..the new progs that droid has ..amplitube and irig for android..
sadly android and iphone both have pretty good real time effects processor apps ..(not as good as rak or jack rack or gtkgep even .. in my opinion ...just shiny and slick)
.and actual gear to hook it up proper...
which realllllyyyy pisses me off to no end...
hm.
well..I'm not desperate enough to use a droid or iphony ..
rather settle for our patchwork throw-together gtkgep until a gtkgep eureka or other prog happens..or a proper linux umpc shows itself capable...
there is my micro rant/edification/illumination on an aspect of this handheld easter egg hunt...