Google adds “magazines” to the Google Book Search index, so that when you search on Google Book Search, you'll be searching across the full text of both books and an ever-growing number of magazines, which will appear tagged with the keyword "Magazine" in search results. Try queries like [obama keynote convention], [hollywood brat pack] or [world's most challenging crossword] and you'll find magazine articles alongside books results. Magazine articles are tagged with the keyword "Magazine" on the search snippet.
If you want to restrict your Google Book search to just magazines, you can do so via the advanced search page.
More info: Google Blog