I love Smashwords, but darn, it's hard to find the books I want there. The UI just ain't good for browsing. I use it to buy books, but I usually find out about books through forums/blogs/goodreads/places not Smashwords. But reviews will help for the folks who are on the Smashwords book page already.
@Mel: 550 copies and zero reviews is not a cool ratio
I think many people will leave reviews if the suggestion is planted well, e.g. the last page of the book, or a "get this book for free but please review" sort of exchange.
@Abigail: I second Tome Keeper regarding book bloggers. Many are pretty keen on posting reviews across several websites like Smashwords and Amazon to promote their own blog and establish their credibility in a niche. I'm saying this from first hand experience because I'm a book blogger/reviewer as well, and my reviews get on my blog, Goodreads, Amazon and Smashwords. Plus, they are linked to through twitter, mobileread threads, the Simon Royle indie book reviews page. Not sure how you've been utilizing book bloggers, but they can be useful in sending little waves of promotion as long as they reach an audience relevant for your book. Little waves sometimes set off bigger waves too.