Samsung Galaxy Nexus can grant its whole screen if an app requests it, hiding away the on-screen keys
Units that are closer to the retail state are making the rounds, and this pressing question has been answered already - as you can see below, videos do switch to full screen mode, which utilizes all 4.65 inches of the display, hiding away the on-screen keys.
This is one of the coolest features of Android Ice Cream Sandwich, as it can finally lead to phones with bigger displays, but the same physical footprint, since there are no front keys to take up space, and hopefully they will come with thinner bezels.
The Galaxy Nexus itself is not a fine example of this desired category, as it is taller than an HTC Sensation XL with a 4.7" screen, for example, although narrower. Oh, well, we know that one still has to place the antennas in a place that will get reception within the phone, but hopefully manufacturers will get creative with ICS phones next year.