On March 9th, Google updated the review-specific usage guidelines and policies for reviews and rating rich snippets.

According to Menashe Avramov, who posted about this on Google+, Google added that these types of snippets must be placed only on specific items, not on category or a list of items landing pages.

Here is the additional text added to the page:

So if you plugin [hotels in Madrid] what do you see? Well, no organic results are in your view and I have a huge Apple Display monitor. You see AdWords ads and Google Hotel ads all containing reviews/rating stars. If you scroll down until your finger hurts, then you will see organic results without any ratings or review markup on the page. Shouldn’t the organic and paid search guidelines be the same for this?

In any event, keep this in mind for your organic implementation of rich snippets around reviews and ratings. Despite what Google does on the ad side, the organic side does not allow it.

Forum discussion at Google+.