Sean Lyndersay, Lead Program Manager on the PowerPoint Web App team, has just put up a post that talks about some of the high level considerations of creating a web-based version of PowerPoint. The post links back to this blog at one point so if you find yourself back here, be careful to avoid getting into an infinite loop. Sean's post is here:

http://blogs.msdn.com/powerpoint/archive/2009/07/24/bringing-powerpoint-to-the-web.aspx

Nick Simons
Program Manager, Office Web Apps