I think you're missing the point of Dave's simple design. The opamp needs a reference voltage which it wants the feedback to control to. It will regulate the transistor untill it reaches it's reference voltage. Because it's a voltage, not a resistance, you can use any DAC you want. The potmeter is used to get a voltage really easily. If you want to digitally control this thing completely (I.e. make a variable load controlled by USB) you could better get like a SPI 10-bit DAC (1024 steps) or a PIC with one.
If you'd use a more cheaper digital pot, like 256 steps, you can only control every 4mA.
Digital pots aren't that accurate really. 25% tolerance (min to max resistance!), and if you take a 10-bit version you'll pay an impressive amount of money (6 euro's here for the MAX5481). And how much is a dedicated 10-bit DAC? Like 2,50 euro for the AD5611, which can change 'value' at 1.7M times per second.
Analog pots are just the best components to adjust and calibrate analog designs, especially when high accuracy is needed. You could use digital pots if you have a schematic that requires a real resistance instead of a voltage to control it. In case of dave's design, it requires a voltage, so anything that produces a voltage (DAC, resistive divider with pot) will work. Then just is the question, which one is cheapest/best/reliable
