The Google Blog announced that they added a link to some books named "Find it in a Library". I have been unable to find a book in Google Book Search with a link named "Find it in a Library," just yet. Google says that clicking on the "Find it in a Library" link will send you to the OCLC Worldcat. At OCLC you can enter your zip code and find a local library that has the book you search for.
If you are looking for alternatives to Google Book Search, I know Gary Price, the librarian himself, is a huge fan of RedLightGreen for book search. Gary wrote a SearchDay article on it named Searching for Library Books with RedLightGreen. Other alternatives Gary wrote about include NetLibrary and ebrary.
Postscript: After researching more I figured that new books would not have the "Find it in a Library" link. So I searched on a out of copyright book and presto, here is an example (see the bottom left for the link). I am not 100% sure this is brand new, the Google Blog post may be just pointing out that this is a feature on some books.
Postscript From Danny: The library links have been in place since 2004, as covered here. But Google says it wishes to expand them and also highlights in the post that if you can't wait for this, this Greasemonkey script for Firefox will do it for you.