With the help of GLaDOS, its turrets, and obviously a whole bunch of portals, new and more experienced bridge-builders are able to solve increasingly hard physic-based puzzles in Bridge Constructor Portal, based on the popular Valve series.
Publisher Headup Games and developer ClockStone Software, which are responsible for the popular Bridge Constructor, are working directly with Valve for this followup. While this certainly isn't a Portal 3, which undoubtedly will leave some fan's disappointed, I think the a full-fledged Portal game on iOS really wouldn't work that well.
Interestingly, this also means that the game will not be Windows exclusive.
Portal and Portal 2 both have players solving traversal puzzles using the titular portal gun. Additionally, GLaDOS singing Jonathan Coulton's "Still Alive" is. Players will explore never-before-seen areas of the Aperture Science Labs and be reunited with GLaDOS, the occasionally murderous computer companion who guided them through the original game. Feast your eyes on these screenshots, JUST LOOK AT THEM!
Bridge Constructor: Portal will be out on PC, Mac, Linux, and mobile on December 20. That game featured a new slice of story with McLain and other returning stars set after the events of Portal 2 - and which is canon in my head until Valve says otherwise.
No firm release date has been announced.