On the other hand a web based presentation creator would be useful - especially if you could create a base presentation and then have little forked versions which benefitted from updates to the main content if they wanted.
By seperating out the content and the presentation you could have it automatically create content in Powerpoint, Keynote, PDF, S5 ... the list goes on.
You could tag presnetation and make them public or private thus giving your clients (or presentation audience) the ability to go find presentations about subjects they were interested in (public) or have access to them later (private). Hell you could even attach Creative Commons licences to them and allow anybody to give your presentation.
I have nefarious designs percolating through my head with object heirachies that would have a presentation being a collection of slides with a style applied to it and slides being content holders that can be versions or branched (go, go VCS backing store).
Oh for more time.