I understand that included in standard contracts is a means to regain your rights if a work hasn't been republished in a certain time. I doubt any agent would agree to a contract saying the author loses their rights forever. Certainly some fiction has been republished a number of times by different publishers. This seems to indicate that it is fairly common for an author to regain his rights and be able to re-sell them.
here is a link to a sample ebook contract
http://www.epress-online.com/contract.htm