I went to Baan Khanitha twice with an Isaan (northe-east Thailand area) native, and she loved the food there. She said that it was the way Thai food was supposed to be, with nicely balanced flavors - and not all salty and extremely spicy, the way Thai street food can be. Of all the restaurants I have taken her too, this was by far her favorite. I like it a lot myself, and I have cooked Thai food from cookbooks written by Thai people. It did not seem to me to be compromised by its clientele, many of whom were Thai.
Maybe all one can say is that if a Thai person cooked it, it is by definition authentic. After that, only evaluations of its quality by native Thais can tell you whether the food meets locals aesthetic standards.
I just found this thread last night! I have been cooking from Dunlop's books for ages now, and love them. I made the Taiwanese version of General Tso's chicken tonight and it came out well. I used 2.6 lbs of chicken breasts (not thigh); half black vinegar, half white; chicken stock instead of water; and added chopped sweet red peppers (fried in advance) to pad it out while cutting the calorie density.
As I live alone, I always have leftovers, so rather than returning all the chicken to the wok at the end, I mixed enough sauce with the chicken I planned to eat tonight, keeping the rest of the chicken separate from the sauce. I think the chicken soaks up too much of the sauce if you mix them together and then let them sit for a day in the fridge.
Incidentally, the sauce tasted spicy-hot, a bit unbalanced, so I ended up adding a little sugar to it to balance it, so maybe it was not a pure Taiwanese version.
As an interesting aside (well, interesting to me!) - the ingredient list for General Tso is extremely close to that for Kung Pao. I was quite surprised when someone at the rec.food.asia newsgroup first pointed this out to me.
Cook on, everybody - this is a great thread!