Historically, Google hasn't counted structured markup as a ranking signal in search results. I've always found this somewhat surprising considering how actively they've encouraged webmasters to use it, but based on what the company has said on occasion, it has not been included in the over 200 signals.
Now, Google is implying that this could change in the future. Search Engine Roundtable points to a new Webmaster Google+ hangout with Google Webmaster Trends Analyst John Mueller, who makes the suggestion (jump to 21:40).
Mueller said that over time, structured markup might be part of the rankings.
"If we can recognize someone is looking for a car, we can say oh well, we have these pages that are marked up with structured data for a car, so probably they are pretty useful in that regard," he said. "We don’t have to guess if this page is about a car."
He said it "definitely" makes sense to use structured data, adding, "So I think in the long run, it definitely makes sense to use structured data where you see that as being reasonable on the web site. But I would’t assume that using structured data markup will make your site jump up in rankings automatically. So we try to distinguish between a site that is done technically well and a site that actually has good content. Just because it is done technically well, it doesn’t mean it is as relevant to the users as content that is not done as technically well."
In other words, it would be just another signal. Obviously.
If you aren't already using structured markup on your site, you might want to start implementing when you get a chance. At worst, you'll have a technically better site. If it becomes a ranking signal, it will help you all the more.
If it's not something you've dealt with, here's a good place to start.
Images via Google