In this context, I was referring to something that redirects packets from
one address to another (in two directions).  You could call it a router,
tunnel, forwarder, proxy, ...

It's a static IP address that just acts like a front end for the GPRS
device.  (If you only need a single TCP/UDP port per device, for example,
it could get much simpler.)

It depends on how much you need to do with it, but iptables, xinetd, or
socat can be pressed in to service to do this.  (Heck, you *might* even
be able to get away with just manipulating name resolution.)  The tricky
part would be notifying the tool of the GPRS device's address.  That
depends on the capabilities of the device (and the security you require).
At one extreme, you could have each device just send ICMP (ping) packets
with their IDs.  The tool would listen for the packets and update the
routing table.  It'd be easy to subvert, but it'd be enough for a demo.

It basically needs to do what you would otherwise have your application do
(as far as tracking the device).  I was just suggesting that you could
modularize the functionality and keep it away from the application.  That
might have some advantages (in terms of maintenance/testing/politics/...).

I dunno.  There are lots of people who could whip up something usable in an
evening.  I'm sure there are many of us who would be willing to point to
helpful resources, given a few more details of the requirements.

BTW, I use GPRS, but I run SSH over it to make it feel more like a static
address.

--kyler