Users of the mobile version of Google Product Search can now see if items are in stock at a local store from select retailers. These retailers include: Best Buy, Sears, Williams-Sonoma, Pottery Barn, and West Elm.
Product listings from these retailers will have blue dots that can be tapped to see an "in stock nearby" link, which takes you to a seller's page where it will either say "in stock" or "limited availability". In addition, these pages will show you how far away the store is from your location (assuming you have My Location enabled or have manually specified your location).
The feature is available for iPhone, Palm WebOS, and Android users in the U.S. Users can hit the "more" link, and select "Shopping" or find the "Shopping results" section in Google's Universal Search results when searching from Google.com.
Google offers a form for retailers who are interested in participating in this program. Google says to make sure you have your Local Business Center data up to date and to ensure your Product Search data is in "great shape."