The black Xbox is a dev kit Xbox 360. ALL the dev kits that the developers get look like that, and they ALL have 60GB harddrives or more, and ALL have an altered OS to give extra feedback and data to allow the devs to see how their game is running.
As for the rumours of a new Xbox 360, they are true. MS are changing the process that is used to make the chips from 90nm to 60nm or less, in May/June time which will result in a cooler and quieter Xbox 360. This revised 360 may or may not have a HDMI output, It is true that a number of 360 units have been produced with HDMI out, put MS have not said/decided whether or not retail units will be made with these.
As for a larger HDD, that is ineviatble. MS said from day 1 there would be other HDD sizes available. The technology is easily there, its just a question of when. And since they're detachable, you'll be able to buy one separate if you so wish, when they come.