According to Hugh McCleod, the increase represents "tens of thousands of cases." This is undoubtedly an exceptional case (being one of the first), but it's worthwhile reading. And to be completely accurate, it also involved giving away a hundred or so bottles of wine, not just one person blogging.
Lots of attention is focused on wine critics (Uncorked and Fermentation both posted recently on this topic), but generating word of mouth isn't solely the province of the critics. By giving away wine to bloggers, Stormhoek generated word of mouth interest in their wine in the UK (which is now coming to the US).
It doesn't have to be blogs.
Suppose you sent a bottle of wine to your biggest fans (notice I didn't say "best customers") and just asked them to pass it along to a friend who is unfamiliar with your wines. Some would, some wouldn't, but your fans would appreciate the recognition (making them more likely to talk about your wine), and some people who hadn't previously experienced your wine would get to form an impression about it. I'm sure that a marketer more clever than I could figure out ways to make sure it got talked about with clever copy and presentation.
One advantage that smaller wineries (the 95% of wineries competing for 5% of the market) have over larger wines that sell primarily through distribution is that they probably *know* who their biggest fans are. To see who your "most effective fans" are, send that free bottle along with a coupon that identifies the "fan" and offers their friend who gets the bottle 10% off any wine purchase.
Who are people who might be willing (even eager) to talk about your wine? Maybe you should send some of those free bottles to restaurants or retailers or distributors (or bloggers, like Stormhoek did). Never forget it is ultimately the experience of your wine that sells it - reviews, 100-point ratings, and even blogs are just surrogates for the actual experience of what comes out of the bottle.