Dirson, a spanish Google blogger, found an interesting link in his Gmail when he opened an email message with a PowerPoint attachment. The link reads "View as slideshow" which opens up a new window with the slideshow that works through your browser. The feature, unfortunately, isn't widely available yet -- I don't see it in my account yet either.
As Tony points out, the technology behind this feature appears to be flash if you look at the source code. That is a bit surprising to me -- I would have put money on an AJAXy application like Writely or Google Spreadsheets.
* SWFObject v1.5: Flash Player detection and embed - http://blog.deconcept.com/swfobject/
* SWFObject is (c) 2007 Geoff Stearns and is released under the MIT License:
Tony also does some more digging into the "Embed this presentation" feature -- he concludes that the service obviously hasn't been fully tested. That's likely why it's not widely available yet.
The embed code is still a little buggy – it currently lists “http://mail.google.comhttp://mail.google.com” in the URL – and doesn’t even appear to be working yet. (Oddly, when trying to load the embedded URL, my browser tries to access Google’s internal www.corp.google.com domain, suggesting they’ve not finished testing this feature yet.)