hello jon
there is a quick way if you're using purely incandescent lights. buy a velleman K8064 dimmer kit. it should cost around 20 pounds.
this is a voltage controlled dimmer. it can be tuned to work from the 0 to 5 v output of the arduino board. remember to place a big capacitor at the arduino output pin because the dimmer doesn't like the fast PWM pulses.
if you want to drive an analogue dimmer using a 0-10v input you just need to build a simple circuit using an op-amp.
what you need is a non-inverting amplifier with a fixed gain of 2. power the opamp at 12v and you'll get a 0-5v coming from arduino into 0-10v needed by the dimmer.
the above mentioned circuit is just made with an LM358 and 2 resistors....
don't hack esisting dimmers... it's way to dangerous to play with 220V. expecially because cheap dimmers have 220v running allover the circuit and they will happily send it back to the arduino while the velleman dimmer uses an optocoupler to separate the board from the 220V part of the circuit
finally DMX is best used when controlling multiple lights in a big installation. DMX fixtures are not so cheap..
have fun
massimo