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.
By default, you have to manually advance to the next slide, but there's an option to start the slideshow.
If you have PPT/PPS attachments in your Gmail account (do a search for filename:.ppt OR filename:.pps), you could try to see if you have this feature by looking for a link that says "View as slideshow" next to a PowerPoint attachment.
Update (June 12): The feature is now available for everyone. The presentation embedding option has been removed, although it could be reincluded when Google launches its full-featured presentation app, in the coming weeks.