Update: The new model does indeed support new hard drives. To access the bay, you need to push off the plastic bezel on one of the system's ends. We apologize for the oversight.
As for the new disc tray, there's a plastic latch connected to a large plastic bar on the front of the console. Pressing the bar on the right side will turn the console on and off; the left side of the bar will release a latch holding the sliding door, allowing it to slide back and reveal the disc tray. Perhaps the saddest part of all this is how the sliding door doesn't make that "whoosh" sound from Star Trek.
In summary: It's indeed lighter and smaller than the previous two PS3 models, and it doesn't make any sounds from Star Trek. None.