Prost are no longer in existence, and Peugeot don't make engines anymore, at least, not as Peugeots. You'll be happy to know that the engine powering Mark Webber is a rebadged Peugeot, though, the Asiatech by Asia Motor Technologies.
UPDATE: Prost has been bought by an unknown venture and is to be backed by Tom Walkinshaw in the tech department. As we speak they are trying to return at Malaysia, but there are some people (namely Stoddart) mounting a legal challenge to stop them. If they return, they would not apparently run Peugeot engines, though.