No, this is not a new release from Valve to continue the great game Portal. But you can imagine how a huge Portal fan like me, was excited to hear about this project.
The game is being developed by a bunch of French Portal fans as an independent, totally-free and non-for-profit release and the hype that goes with it, is certainly well deserved, given the teaser they posted on the official website:
Portal: Prelude, as its name states it, is an unofficial prequel to the game Portal. Its story revolves around the pre-GlaDOS epoch, even before she was plugged in. At this time, test subjects were monitored by real Aperture Science employees whose work was tedious, lengthy and repetitive. This is why they decided to build a great artificial intelligence that could both replace them in these difficult tasks, but also take responsibility for many other tasks within the complex and compete with Black Mesa's superiority. All employees of the Aperture Science complex are now eagerly awaiting GlaDOS. Maybe even a little too eagerly, as the upcoming events will tell...
The game development is not being supported by Valve, but they know about it and don't have a problem with it, as long as these guys don't make money out of it :-)
So, for now, all we have to do is drool while watching the trailer and wait for the release... but let's hope it won't be long.