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!