Valve spokesman Doug Lombardi said the company would be pursuing the Portal phenomenon, and that it planned to add significantly more to the game that simply "a bunch of new puzzles." Speaking to Eurogamer, Lombardi said, "There’ll be more Portal, for sure. But the details of that, to be honest, we’re still working out."
"The only smart thing we did for Portal was to hire those seven kids and give them the support they needed to make the game," he added.
Typically for Valve, Lombardi wouldn’t comment on any sort of potential release date of either Portal or the next chapter in the Half-Life 2 story. "With Episode Three we want to live up to the promise of where we are taking things to; there’s a lot of working being done to make sure we deliver on that promise," he said.
Portal, a short puzzle game that was released in 2007 as part of Valve’s Orange Box, became an instant sensation for its unique gameplay, closing credits song and the Weighted Companion Cubes. Despite its brevity, the game has garnered a Metacritic score of 90, and is the first and apparently only game to ever receive no criticism whatsoever in Ben "Yahtzee" Croshaw’s Zero Punctuation.
Source: The Escapist Magazine