I've been working on my own idea for a digitally controlled power supply over the last couple of weeks, originally using a MIC29302 which didn't look too bad, and a handful of DACs and ADCs to go with it, however I abandoned that project when I realised that it would cost me over $200 just to get the parts/boards. So I headed back into the LM317 route again.
Following some of Dave's designs I came up with the attached Schematic so far. Most of the important info is there, apart from the Opamp needing to be powered from a voltage rail that goes up to 1.25V under the Maximum output voltage you want, and down to a minimum -1.25 so that you can get true 0v output.
The question mark next to the input pins on gate A is there because i'm not 100% certain on the orientation, so I'll have to try that out on a breadboard. I think I might need to swap them to have it work right. (and the more I look at it the more certain I become)
I'm also not 100% what voltage Vref should be at, but again, that will require testing on a breadboard.
The current limiting and voltage control have a number of ways that they could work. A pot, DAC, uController PWM with a filter, would all be options.
If anyone wants to make a comment on the design, please feel free to.
-kizzap
Edit: Doing some more thinking, I'm not entirely sure about where I have the current limiting connected at the moment either, I might have to place a resistor between the output of gate A and the LM317 adjust pin, and move R4 over to there as well to create a voltage divider there.