Here's what it looks like:
It looks like the same technology that the 'View Presentation' option for Powerpoint slides in GMail is based on, which is more than likely the same technology that Presently (the Google Powerpoint) will be using.
Here's the anouncement from Google.
Update: In related news the Google AJAX Search API blog has also announced a fancy new tool that lets you add a similar slideshow based on any media RSS feed. So now you can embed slideshows for PhotoBucket, Flickr, or any other RSS photo feed right in your blog or site. Nice!