interfacing with Devireg 550 Thermostat

wondering has anyone tried/succeeded using Arduino for interfacing with Devireg 550?

Theoretically could imagine it should be easy to replace the high-cost (200+ EUR) Devicom Interface A serial interface with Arduino based solution (original software available for free here).

Apart from everything else and cost savings for being able to program the sensors across the rooms, with using Arduino could also implement plenty of other sexy features (e.g. schedule heating on morning only if presence sensors detected somebody was at home on evening/etc).

Even if nobody tried - any good ideas where to start digging from?

cheers, austris