How do I explain 2+ years of unemployment on a resume?

I just finished a temporary (3-month) job that paid under-the-table. I have been unemployed since November of ‘07, with the exception of a few temporary assignments. I am updating my resume/CV, and I am anticipating that HR managers who see such a long period of inactivity will be more inclined to pass me over for someone who has been working. Is this an unfounded fear, given the economy, or do I need to come up with some explanation in a cover letter?