I've run into this problem before with both PPT files and PDF documents. The solution, actually, was quite simple ...
There's a plug-in called PDF and PPT Viewer that will uses Google Docs to render your presentation inside the page. Each slide/page is rendered as a image using Google's API rather than Flash or a native file viewer.
I'm using this extensively on my portfolio to show PDF files without requiring a file download or browser plug-in. Here's a specific, live example of a press release I did in January: http://work.eamann.com/2010/02/publicity-release/.
Note how the plug-in embeds the Google viewer in an iFrame on the page. The file itself is hosted on my own site (I used the WordPress media gallery uploader).
Important: The plug-in download page claims the system is only tested through WP 2.8.4, however the site I've referenced above is using 3.0.1 ... it is compatible with the latest release of WordPress, the author just hasn't updated the documentation yet!