It appears that the new feature is only live in the US as we cannot, despite trying a wide range of searches, see the new results formats but the fundamental change is that more results are going to be show for the same domain for some searches.
The example that is used on the webmaster central blog illustrates how it will work and is worth looking at to understand the principle.
The reason that we are not convinced by the change is simple, you can already quickly see more results from any specific domain by using the ‘show more results from’ feature as highlighted below:
By having seven or more natural listings from the same domain on the first page of Google’s SERPs, we believe that it is likely to cause user frustration and it does not really help in any way that we can see.
It is also likely to cause some real pain for sites that are currently ranking in the top five natural listings who may suddenly vanish off the first page entirely.
Of course, there will be winners as well as losers and the impetus will be to ensure that yours is the site that is chosen to dominate the natural listings but we just don’t believe that it is good for one site to dominate in such a manner.
It is not clear whether this is a permanent feature or whether it will migrate across the pond to our shores, but we would be very happy to see it moved into the ‘nice try but ultimately a fail’ box.