- I have a spare out on the dim controller - if I dim to 70 or 90% then I could feed my Arduino with a steady 7 or 9v (good or bad idea?)
Bad idea. The output will most probably be pulse-width modulated, not a steady DC current.
- I think I need to connect to ground of the dimcontroller anyway if I want to measure the voltages on the controller (all grounds need to be connected?)
- I don't want to consume more energy measuring the dim controller output than the lamps consume (nor do I want to set my house on fire) so what are sensible resistor values for the voltage divider and which wattage should they be?
You need to take into account the input impedance of the analogue inputs as well as the current consumption. Have your resistors too high and the readings will be wrong, as the ADC will form part of the resistive divider. Have them too low and you will waste current.
Resistors in the region of 10K are usually fine. Say you have a 50/50 divider, at 10V, with 10K resistors - that's a 20K resistance with 10V over it, which gives 10/20000 = 5µA - pretty much nothing.
You might want to low-pass filter the signal first if it is PWM so you get a more steady DC signal for the ADC to sample, rather than a rapidly changing square wave.
Also - if this is a stupid idea I can accept that - I've always found the programming side of the Arduino easier to understand than the real world electronics.
Most of it is perfectly reasonable. Just forget powering it from the dimmer output ;)