The Spanish blog Dirson found a new feature in Gmail that lets you view the attached PowerPoint presentations as a slideshow. Until now you could only download the attachment or view it as stripped-down HTML file.
It's interesting to see this new feature, not long before Google launches the promised presentation tool for Google Docs. The slideshow included in Gmail uses Flash, while most Google tools use AJAX, and seems inspired by Picasa Web Albums.
Among the features, there's an interesting "embed this presentation" that should let you include a presentation into a site. But it's not functional yet.