Oops, seems I got that wrong. I checked with a few other users today after the new UI appeared on Google Maps for me. They were also seeing the new design. However it seems from the comments that Google have just extended the size of the test group.
The biggest change is in the appearance of the map buttons. The buttons have been changed to larger thumbnail images. In the new design when you roll over the satellite thumbnail the layer options appear in a drop-down list.
The list of your recent map searches is also displayed in the layer panel.
Hat-tip: Street View Funny