The exit to the Cub grocery store on Lake & Hiawatha in Minneapolis has a good example of opt-in phone book delivery in practice. Notice the neatly stacked books on the bottom shelf of this display. Thousands of people use this grocery store every day and can easily toss one of those books into their cart on the way out to their car if they so choose.
Creating similar distribution partnerships with other grocery stores, pharmacies, etc. would virtually guarantee that every consumer interested in receiving a phone book could conveniently pick one up during a regular trip to the store.
Opt-in distribution in this form seems to work well for many other types of media, such as classifieds magazines, real estate mags, and alt-weekly newspapers. Surely, it could be used by phone book companies as well.
Imagine how much money phone book companies would save just in distribution costs by focusing on high-traffic locations rather than going door to door?