I have done a little research in the Adsense Help Center and it looks like
you can place Adsense Ads in password-protected websites. However, if you want the ads to be targeted you will have to complete an extra step. You have to give the Adsense Web Crawler your password. This is done using a feature called Site Authentication.
You can read how to do it on the Inside Adsense at http://adsense.blogspot.com/2007/07/more-sleep-for-adsense-crawler.html. I think this is what you are looking for.