One complaint that has been consistently leveled against in-app purchases is that it's easy to accidentally make a purchase in real-world money by accident. In-app purchases, which can include anything from extra level packs to extra play time or ad removal, can often generate more revenue than the initial app purchase. With previous iOS versions, in the first fifteen minute period after downloading an app, in-app purchases can be made without having to re-enter you password. The Washington Post writes that parents had complained that, "in the 15-minute period after an app was downloaded, children were buying sometimes hundreds of dollars of purchases on games such as Smurfs’ Village and Tap Zoo — popular iTunes games that are also among the highest-grossing programs for in-app purchases." In the Smurfs app, for example, a barrel of "snowflakes" or "Smurfberries" can run as high as $99. Besides, "fat finger syndrome" can also lead to unintended in-app purchases.
In response to such complaints, Apple has changed its handling of in-app purchases in iOS version 4.3. Now, a password will also be required to make an in-app purchase, though for fifteen minutes after entering your password that time you'll be able to make multiple purchases. So, it's a small hassle but if you're making lots of purchases at once it shouldn't be too bad.
Of course, you can also just switch of in-app purchases in the "Restrictions" section of the Settings app if you're really concerned about your kid spending too much money of Smurfberries. At the price of a small inconvenience, this new setting should prevent some parents from getting too irate, but I do wish that Apple had made it an optional (albeit default) setting that we could tweak personally.
If you're anxiously awaiting a means of preventing accidental in-app purchases, updating to iOS 4.3 should solve your problems.