The "usual" mechanism to talk to the outside world is to set up a local web server and grab XML data from that.  The iPod support uses this mechanism.

Another option is to use Pipe Open to open up a driver file, and talk to it that way.

PipeOpen is discussed here: http://forum.chumby.com/viewtopic.php?id=3125

And I have an example of how to use it here: http://forum.chumby.com/viewtopic.php?id=3623

ok
i've had this working now using the following code:

pipeArduino = _pipeOpen( "cat /dev/ttyUSB0", "r" );

function fnArduinoRead()
{
    s = _pipeRead( pipeArduino );
    if (s != undefined && s != "")
    {
       sarr = s.split("\n");
       status.text="";
       for (i in sarr)
       {
           status.text += "[" + i + "]=" + sarr[i] + "\n";
           if (Number(sarr[i])>0)
           {
               fInput = (Number(sarr[i])/1023.0 - 0.2)/0.4;
               mcBG._alpha=100*fInput;           
               trace(fInput);
           }
       }

    }
}

setInterval(this, "fnArduinoRead", 40);

there's a seperate as file with all the asnative links:

_pipeRead = ASnative(5,193);
_pipeOpen = ASnative(5,191);
_pipeClose = ASnative(5,195);
_PlayAudio = ASnative(5,151);

this worked fantasitcally (i had one variable coming in over from arduino as a decimal)
but after rebooting chumby
i'm not getting anything on /dev/ttyUSB0 anymore
when i run

cat /dev/ttyusb0

i get no response
if arduino isn't plugged in then i get told that it doesn't exist
if i do

cat /dev/ttyusb0 | less

then i get loads of '@'s, constantly coming in
also, i notice that when cat is running, the lights on arduino spark up showing that transmission is happening
but nothing when cat is closed

any idea why i wouldn't get anything out of cat anymore?
when i connect arduino to my laptop i still get data coming out
i think maybe i need to set the baud rate etc on the chumby?

hmm, seems that (at the moment) if i dont have arduino plugged in when i start up then it's ok
if i have it plugged in on startup, and replug it in, then same problem
so.. any idea what's up?

hmm, after that i still have gettywrap running when i check ps
and cat /dev/ttyUSB0 doesn't give anything
/etc/inittab is read only
is there a way of modifying that?

If you didn't mind also having a USB dongle plugged in at boot, you could put the following in a file at the root of the drive called "debugchumby" and leave it plugged in at boot:

#!/bin/sh
touch /tmp/ttyUSB0.nogetty

I believe (though haven't tested) that this will get executed before getty starts up.