Saw this on Steam news today and found it pretty interesting:

For five months in 2008, Valve Software was making a version of Portal 2 that wasn't going to include Portals. And it was going to be a prequel, set in the '50s. The game, based on an internal prototype called F-Stop, was built around a new gameplay mechanic that thrilled people at Valve.

But the feedback from test players was harsh, according to journalist Geoff Keighley, who reveals the story in "The Final Hours Of Portal 2," a slick multimedia app that launched today on iTunes. - SOURCE
Of course, it was called F-stop because of it being Aperture Science. Clever I think.

Beyond that though, this sounds like a very interesting concept. We haven't been told how it works, but I'd definitely like to see Valve demo is at some point, especially if it thrilled the people at Valve.

Overall I think they made the right decision though, as the pre-existing storyline was good enough to build on quite significantly. I expect we'd all have been a bit miffed if Portal 2 was something completely different. I love Portal 2, but I expect that they won't try to extend the story and further. However, I wouldn't put it past Valve to go back in time with this prequel and make a Portal 3.