As I've said before, I think this is going to be a different story because, while collaboration brings a lot of value to presentation software users, I can't see a killer app that Google can leverage beyond Image Search that would make their product a must-have. Zenter has done a great job making a connected presentation app with a very slick Google Search-based image import tool.
Pound for pound, I think Zenter has a fighting chance as long as it keeps innovating on the feature front. Don't forget that collaborative presentations will likely give rise to a slide sharing community (i.e. slideshare.net), so there's more to online presentation software than the app itself.
I'm not sold on how Google can leverage their offerings (aside from better image searching) to make as strong of a case in presentation software as they did with online calendars.
Using the same calendar system others use helps. Using the same presentation system mostly doesn't.