Nice one, and real nice looking modular, I take it your running all outputs direct from the arduino?
I did the naughty for the oscilloscope and sent it through a pre-amp before my computer btw, the signal was very low anyways, but it enabled me to run an oscilloscope app too get some rough idea what was going on.
I'm sure some people on here will caution me on this, as I'm sure I should be using some input buffer of some sorts :)
I'm trying to conserve ports on my arduino as to get midi input also, and possibly another serial in for some crude control input.
Running it through the hc595 causes a bit of spiking due to it not holding the signal I think, Im now experimenting with some gate- flip/flops to see if i can do sample-hold the signal before it goes through the R2R.
It would all be a lot easier for me using something like the DAC0800 like you mentioned, but it isn't half as much fun and messy as the R2R is it ? :)
Project is up here along with a polydac build if your interested.
Theres also an arduino page on there where I have a rough arduino pdf and omnigraffle stencil I use for mocking up some ideas.
I'm probably going to build the arduino into part of my modular as shown somewhere earlier in the blog. In theory once its running the polydac should be able to through midi to the arduino rack while the arduino outputs sound to the rest of the modular stages. I'm hoping for lots of noise and character :)