First off: thanks for the replies! Wow! ALUG'ers do it again! :)
Second: I'm investigating this problem for a uni project (for my MSc), so I guess the ALUG archives are going into my bibliography! ;) so, umm... these units are *fictional* lol, sorry! :)
Third: I kinda thought on scrapping the RS-232 thing, since I'm fairly comfortable with ethernet and TCP/IP, but since these units have rs-232 (and they were duly mentioned by my tutors) I thought "maybe there's a reason ppl still linger with it". My first idea was to have two ipcop (www.ipcop.org) servers(firewalls) at either end of the connection to sort out a vpn over the POTS. Luckily we are dealing with temperature ranges of 5-30, so there's no real safety issue that can't wait the time it takes to dial up!
Fourth: 'tis my job to write the programs to handle this, so that's all cool.
Well... of to do some planning and decide whether SSH tunnels or a VPN would be better in this case.
Thanks! Ricardo