In the old days google saw a hyphen as simply a space. So for example if you had a site about blue bunnies the site bluebunnies.com would not return as well as blue-bunnies.com for the search term "blue bunnies".
This is because blue-bunnies.com was seen as two seperate words, and bluebunnies.com was seen as one.
However it appears this is not as big of a factor anymore. google is able to seperate commonly used words better.
For example, when you search for my commonamerica.org site using the term "Common America" i return first:
http://www.google.com/search?hl=en&i...=Google+Search
However if you mix up the terms some you will see that this is not the case for other things:
http://www.google.com/search?hl=en&l...ca&btnG=Search
http://www.google.com/search?hl=en&l...ca&btnG=Search
for a few examples of this.
Being a new site the only real advantage i have over other sites, such as Dean for America which i assure you has far more links than I do, is the domain name. There are even other sites with the words Common and America in the title. Google is able to divide the words common and america up that are in my site address because they are both commonly used words.
All that being said, i feel hyphenated domains are beginning to lose any real advantage that they once had over the un-hyphenated domains.