It's not clear exactly how widespread the problem is, but it looks like at least some Dell Mini 9s are shipping with smaller batteries than they should be -- a situation that, you may remember, also plagued some Eee PCs a little ways back
. According to jkkmobile, while the all the batteries are labeled a 32Wh, 4 cell batteries, a quick software test of their own Mini 9 revealed that the battery was in fact only 24Wh, which certainly cuts into the promised four-hour runtime a bit. Apparently, the folks on the MyDellMini forums were able to surmise that batteries with labels beginning in "CN" are the proper 32Wh models, while those that begin with "KR" are the smaller ones. Of course, there's no way to check that until you actually get one in your hands, so we're hoping Dell steps in to clear things up a bit.
Update: It looks like MyDellMini forum member that initially reported this issue has since run a few more tests and discovered that the two batteries aren't as different as it seems at first, and likely not different enough to worry about. That's further backed up by some rundown tests, which actually showed the supposedly smaller KR battery lasting a bit longer than the CN one.