Thanks for the recommendation smudge
@ Bez - I'd love to help out, but don't have the time to make custom setups for everyone's needs. What you want is possible to make, however, if you already have a dynamo wheel and a "low beam" light but no great surplus of spare time, I'd recommend getting yourself the new Exposure Revo dynamo light. You could use a mechanical switch to cut the power to the light from the hub, which would turn the light down to running on the minimum standlight power alone (which I believe is a around about half power), to give you a "dip" mode, but still leaving you enough light to see potholes, ice, puddles even when moving at speed. You could even switch the power between the Exposure Revo and your current commuter light, should you wish to run both lights to give you a choice between one or the other.
I don't believe the Revo, as I have tested it, has a full kill switch, but I used an on/off with the main power (which I spliced into the light myself) to do the same as it was clear that it was too bright to shine at car drivers on full power, unless I tipped the beam downward significantly. I now no longer get flashed by oncoming traffic!