Over the years we've come up with a few clever hacks to speed things up. For example, we keep our designs in svn and can quickly push updates live to our servers using a commit hook. But this is all sort of complicated, and I'd much rather use a web-app like this.
Dropmocks is very close to being the perfect tool for our workflow, and I suspect many other design/photo studios would feel the same way. A few features that folks like us would want/need: ability to pw protect galleries, ability to maintain revisions of designs, commenting (by both designer and client), and a way to export images to PDF.
Not sure if you'd target Dropmocks for businesses (such as ours), or if you'd like to have it open to the world and ad-supported. If the former, we're the kind of people that would sign up.