From the DNC's own web site.

Submit Your Slogan

We'll go through all of the slogans you submit, and the winner will have his or her submission appear on a billboard in Rush Limbaugh's hometown of West Palm Beach, Florida -- and receive a free T-shirt featuring the winning slogan.
This was talked about on Rush's show today, and he asked one thing, go there and make a butt load of suggestions to piss them off, like "Rush Rocks the DNC"