A WebmasterWorld thread has discussion around the META description. There is no doubt that most search engines look and use the META description on some level. Google and Yahoo and Live Search may use it for your search results snippet, the text under your site's listing in the search results (illustrated here). But does that META description actually have an impact on how well your page will rank in Google for a specific keyword phrase?
Most people say no, the META description will have no direct impact on you ranking higher for a keyword phrase. I honestly do not know the actual value in the META description's power to help you rank better.
While there may not be a direct impact in ranking improvements, as g1smd said at the thread, it does "give the appearance of a ranking increase." How so? Well, one thing the meta description will do for you is help ensure you pages don't look like they are all the same page (i.e. supplemental index) when doing a site command search. More on that over here.
Google also recommends using the META description on your pages.
But does it impact your rankings directly? Hard to say.
Google has a whole section in Google Webmaster Tools, under content analysis for "Short meta descriptions." In that section, Google says the "Meta description information can give users a clear idea of your site's content and encourage users to click on your site in the search results pages." Google then links to more information at this page.
Forum discussion at WebmasterWorld.