The 250W should handle 300W comfortably anyways, so I wouldn't worry. The only major difference I am aware of is that the placement of headers is different than in previous generations of the PSU.
I took another look at their site and it lists the 250W DC-ATX as 300W now with a pre-order ship date of Jan 5th.It's probably the 250W, the 300W DC-ATX hasn't been released yet because the 300W AC-DC isn't ready yet.
So the width has increased a bit to allow for more heatsinking. The final dimensions of the 300W AC-DC adapter are 62mm x 41mm x 146mm (WxHxL).
And Larry said I should be getting a review unit sometime in February so presumably retail release will be around the same timeframe.
You'd really have to mod the interior shell with some cutouts to make it fit properly.
I've already emailed him about this. I'd personally prefer to see the unit ship with a big ol thermal pad with the intention of sinking on whatever case it is in.
As an alternative how would people feel about another 4 mm in height? I would think lengthwise fins over 56mm in width would dissipate approximately the same amount of heat as the two sides.
Alternatively you could just file the cooling fins on the side down until it fits and save your case.
Looking at the detailed spec drawings, the fins add 4mm to either side. You could hypothetically take the unit down to 54mm without compromising the inner structural part of the case. I find myself wondering if you could just place a thermal pad underneath and make up the difference in heat managment by heat sinking the case.