iOS 4.3 Changes In-App Purchases to Require Password
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.