Hi Bonusmom- we tried the yoga too, to no effect. Have you considered martial arts? That, and swimming, both seem to have helped both my kids (though it is hard to sort out how much of the improvement is just general maturing/development of self-control, etc.) The benefits we have seen include physical (better coordination/balance, strength, stamina, etc) and mental- a big improvement on controlling impulses for DS, more self-confidence for both kids, emphasis on positive thinking, etc.
Physical activity in general seems to help, and on the (thankfully rare) days when we don't have some exercise, I can really tell the difference.

Because DH and I both work full time, the boy has an 11-hour day at school. They are allowed some time in the gym both before and after school, but I don't think the kids spend very much time being active (they also get their snack in the gym right after school, so I think there's a lot of time spent sitting at the table, chewing the fat while they're chewing their pretzels). I just don't think a Saturday-only martial arts class would be enough. He *loves* swimming, too, but for the same scheduling reasons I just don't see how it could work. I've even thought about giving him a calisthenics routine to do before we leave the house every morning, but he seems to find that a little awkward and resists it. Maybe it's time to try that again.