Thank you all for your posts. Our pediatrician is pretty convinced this is behavioral, though she seems perplexed at the bouts we have of this every-hour-urination. We've had her urine cultured and it doesn't show excessive sugars or anything to indicate diabetes. She doesn't even drink all that much. She has had one very very bad UTI (fever of 104+, vomitting, etc.), but the one other time I was sure she had a kidney infection because of the days of accidents, the backache, and the vomitting, I took her to the ER and they found nothing.
One suggestion she had was that DD1 is an inefficient eliminator (I believe that was the term). The theory is that because this is the ONE thing DD1 has always been difficult about, perhaps she has always held back a little when she peed, therefore getting herself off the potty too soon. I'm not sure I buy that, because often we find ourselves annoyed by standing around waiting for her to FINISH ALREADY.
Yesterday, for example, we went to the park around 11am. She peed right before we left. We got home around 1:15 and I suggested she go right to the potty. She said she didn't have to go, and since this often becomes a huge control fight, I suggested that she read one book (we were outside with a pile of library books) and then go. She said ok, but she didn't have to pee. Then, five minutes later, the book was done, she stood up to walk the 20 feet to our house and said, "Mommy! I have to go REALLY BAD!" and raced into the house, just barely making it to the bathroom. While we were gone at the park, she drank perhaps 4 oz of liquid, and ate maybe a little bit of fruit.
That said, she's had no accidents in about a week (except for night time, which I don't count). This always happens -- I think I should take her to a specialist, and then she improves and I think maybe I'm overreacting...