I don't have much experience with prototyping-with-PSD...how standardized is that process among designers? This is a neat tool but it seems like it might be very dependent on good practices by designers, and seeing how adherence to best practices -- in naming conventions, file types, layer arrangement, etc -- can be highly variable among coders, I can only imagine what it's like among pure visual designers.
Large web consulting forms expect the visual designer to work this way, to drop a PSD that they can then convert to a template for their CMS… But working in this way you’re relegated to a role of providing a visual look, not really designing the interaction of the site…
Also on a technoligical level it becomes harder tot exploit the possibilities of javascript etc. if you think in a static fashion. if you want more influence you either need to work in a more ping-pong fashion with a developer, or develop yourself.
In both cases I find myself rarely using Photoshop, preferring an iterative process, with sketches on paper and in HTML.