Let's see, Superbowl's on a Sunday, so you get to charge time and a half, right?

Defining the nature of the deliverables is definitely something that should be in the original agreement. If a client wants to modify the agreement to include handing over source files, that should be an additional fee. That will compensate you for possibly revealing confidential techniques (consider how much you'd want to be paid to teach a private lesson) and for potential lost income on revisions.

On the other hand, there are times when you don't want to be bothered making revisions later on—if the original job did not pay well enough, for instance, or you were working for barter. In that case, I should think that a layered Photoshop file ready for minor revisions would be something you should give away. I did some work in exchange for a cut of music a while back (the only map commission I've yet taken), and since it would be difficult to work out a fair exchange for revisions, I gave the client a PSD that included the base map, a layer of roads, a layer of icons, and a layer of labels. That way, if he wants to change something it's fairly easy for him to do so without having to contact me about it. My long-term commitment to that project is merely archival in case he has a data failure (which actually occurred recently, as it happens).