I have used Arduino connected to a FriendlyARM ARM7 board (firewalled and SSL enabled web server) running over the FTDI using serial commands and can report it works very well. Theres no reason that RasPi and debian also wont work well - worst case is some of the libraries might need a native compile.
The advantage of the Arduino as an interface is the existing hardware support (e.g. dallas 1-wire devices, RF, IR, stepper motors, servos etc) in addition to the advantage that if PI needs a reboot it shouldnt affect the arduino and vice versa.
I have posted quite a bit of info about using a PC/ARM board and Arduino for home automation in the topic http://www.raspberrypi.org/for.....automation
I like the arduino for its simplicity, thats not to say RPi cant deal with the IO side of things (I dont know the capabilities yet as I still have to get my claws on one!) , however I am more inclined to use the Arduino for the HW control (and the Raspi for the processor intensive tasks such as advanced logic/AI, GUI, Server etc. In much the same way I use my PC now to control the Arduino.
I think its a case of using what your familiar with ultimately! there are several ways of doing things and sometimes all of them can be right
To make things even more complex Arduino are releasing an arm based board as well (not sure of the specs)!