If Udo is standing entirely over the black bit and you press Up, he turns and walks into it. This is accomplished by a little walking animation from behind and by gradually fading Udo's diffuse colour to black. Looks pretty good.
The doorframe is a seperate graphic to the door itself, which is currently being rendered with a diffuse colour of 0,0,0. This means that the door graphic can be of a solid door of some kind, and by fading the diffuse from 0 to 255, the door can fade in and out, making it appear to open and close in a fairly efficient way.
Doors are going to link levels. There will be at least one at the end of every level to finish the level, but also perhaps other doors in hard to reach places, or doors that require puzzle-solving to open, that can link to bonus or secret levels.
Just seemed a bit more flexible and interesting than running off the edge of the screen to end the level, and it forces you to end the level at a specific point so that if we implement some kind of special power for flying, you won't be able to finish the level just by flying along the top of the screen.