Silvery - 3:32 pm on Mar 22, 2010 (gmt 0)
In U.S. copyright law, the concept of "brief quotations" is usually considered a "fair use". However, wholesale scraping of hundreds/thousands of reviews from another site and redisplaying them would likely fall well outside the concept of a "brief quotation".
A second consideration might be adopting some copyrighted work, and then making substantial alterations to it. At some point, the changed work becomes a substantially new work, and the copyright chain is broken. However, in the case of reviews, this might be very hard to accomplish.
Don't be fooled into thinking because Google is doing something, it must be okay for you to do! If you're referring to the reviews that Google displays in Google Maps with business listings, I think in every one of those cases Google and the review site have signed content distribution agreements. That isn't done without permission.
Within regular search results, Google and other search engines do indeed spider sites and display brief snippets without obtaining express copyright permissions. Most consider this to fall within fair use, and the general practice is accepted. A general index or guide to other websites, with brief quotes, is a different beast from extracting the primary/core Intellectual Property content from a page and redisplaying it for the purpose of making $. Even the spidering and display of snippets is sometimes criticized
(Newspapers have criticized Google for doing this, although one is somewhat stumped as to how newspapers think people will really find/discover their content -- and one is also stumped as to why a newspaper that's offended by this does not make use of the universally-accepted protocol for opting-out of spidering by setting up a simple disallow in robots.txt.)