We just caught up with Greg Sullivan, senior product manager for Windows Phone, who was showing off a Lumia 900 running a non-final Windows Phone 7.8 build. The WP7.8 build brings the new WP8 home start screen to existing WP7.5 devices, and possibly some other unannounced features depending on the device. Microsoft is being relatively tight-lipped with any of the other details about features that might be coming with WP7.8. 

Anyhow, the new WP7.8/WP8 home screen replaces the previous single wide and double wide tiles with a more dynamic resize option. There are now three sizes, the single wide, double wide, and a much smaller square shortcut. Long pressing brings up the familiar hover and rearrange menu, and tapping on the resize button cycles through available tile sizes. It's shocking how much more dynamic having the smaller tile size makes the start screen appear.