Yep, it´s up and running! Gmail is now letting you view slideshow presentations containned in emails online.
I checked on docs.google.com and there´s nothing there yet to let you make your own presentations.
This is how it looked like in "view-mode":
This was a funny presentation that basically contained pictures.
I tried to see a presentation in an older email I received but the "view as slideshow" link did not show up *weird*.