Right...I own a publishing company, and I've been published by a couple of big publishing houses (Pocket Books and McGraw-Hill), and there's a very big misconception here about how the rights work.
When you create a work, a copyright is automatically created. This is protected under international law by the Berne Convention.
Unless a publisher is being very predatory, or you are writing licensed fiction (such as my e-book with Pocket Books, Diablo: Demonsbane) they will never purchase the copyright from you. What they will purchase are called "publication rights," meaning that you still own the copyright to the work, but they have your permission to publish and distribute it.
So, a contract would usually have a rights clause that would read something like "The Publisher acquires first English-language publication print and electronic publication rights." And, after this edition has run its course, you will be able to sell the reprint rights to another publisher, if you so wish.
