Various microbreweries make chocolate stout that does indeed, to varying degrees, taste of chocolate. Does it really contain chocolate, and how is chocolate used in the brewing process?
It depends on the brew really; some do and others do not. More often than not, the chocolate flavor comes from the techniques used to roast the malts rather than chocolate itself. Some brewers will add additional chocolate to enhance the chocolate flavor a bit, but it generally doesn't get its flavor primarily from the chocolate. The same is true for coffee stouts.
As a side note, I've recently tried a smoked imperial IPA (Runaway Smoked Ferry Imperial IPA) from the Port Jeff Brewing Company, a local microbrewery. I swear to God, it tasted like bacon! But there was not an ounce of bacon in it as confirmed by the brewmaster.