No, once you save as jpg it, all the layers are flatten (merge into one).
The rest who say can unlock background is talking about a different thing. When you open an image, it is by default called a background layer. This background layer is "locked" and you are only allowed limited editing. To "unlock" it you make a copy of it and then edit on that. I think they probably misunderstood your question. Sometimes happens in CS as some folks just tend to write w/o thinking much.