Emailing it to yourself will work. I did it with a 14 page PPT that I created today. It amazed everyone. Great little preview.
Animations will not work on the iPhone as well, PPT wise.
If you are emailing multiple PPT slides to yourself, suggest that you do no more than 2 or 3 PPT files per email. There is a tendency for it to hang if the email is too "weighty", in my experience.
Maybe they should have a Preview.app as part of the iPhone/iPod Touch. eBooks, PDFs, .ppt. would all be nice. Compile that with folder syncing in OSX and we have a winner!
DataViz is currently in the process of developing Documents To Go for the iPhone and iPod Touch. It should be out by the end of this summer, I believe.