It has a 0-10v dim input. Can I just take a 10k digital pot and dim away?
Are digital pots fast enough for "switching" as well?
Digital pots tend to be very fast, fast enough that your eyes wouldn't be able to respond as quickly as they can switch. It's unclear what the input requirements are for your selected dimmer; I could find the information easily from the site you listed. My guess is that the input to the dimmer is high impedance (as in a MOSFET gate), and that the NPN controller you showed a scematic for would be just fine if all you want to do is switch on and off. You could also use a MOSFET for the transistor and virtually elimate the load from the Arduino (presumably the source of your 0-5V pulse).