The guys over at PlanetHalfLife.com have scored a great interview with Portal developers, Kim Swift and Jeep Barnett. They talk about the original concept and how they came to Valve, the developement of Portal, and more. There are also a few never seen before screens, and a new gameplay video!

Quote:
Jeep: Exactly. It allows user input at any time to change the physics system. It was a big issue, but ... we did it (laughs).

Pratt: Yeah, when I was doing ... "15" I think it was ... I had to find enough of a drop in order to be able to force myself over the wall - so I literally had to make another beginning and end point on my way down. I had to sort of snap around and do that.

Chief: And I did it a completely different way where I went up the side of the wall.

Kim: And that's what's fun about our game - It's really interesting to watch how people solve each particular puzzle. Because it is so open you can place Portals almost everywhere. Everyone solves the puzzle in a different way.

Source: http://planethalflife.gamespy.com/View.php?view=Interviews.Detail&id=80

EDIT - Youtube link to gameplay video with PHL guys:
http://youtube.com/watch?v=gdIgiu2Hh-w