I have to laugh about this - we were out shopping for basement stuff this weekend and lo and behold, we carted along a training potty! I've never done this before, but it really worked! We did give it a good try having Nathan use the store bathrooms, but to no avail. So each time Jeremy ended up taking Nathan out to the van to use his own private stall.
We did have an exciting success, though. At church there is a little kid-sized porcelain, flushable toilet in the nursery. It's really quite cute. Anyway, at church I took Nathan in there and he was so excited about the potty his size that was like Mom and Dad's that he just couldn't go. He loved the room it was in and spent all of his time admiring the walls, the light, the light switch, the funny little potty, the sink his size, and so on. He loved it so much that he forgot he needed to go. So, about an hour later, he said he needed to go again, so Jeremy took him back to that lovely little room and Nathan went and hopped right on that potty and went potty. HOORAY!
Then later that day we were at a birthday party and he actually went two times in another big potty while we were out. Jeremy told him that it was just like the other potty in the church and he went and hopped right up and went! We'll see if that solves our public toilet problem for the future. :)