Internet Kraken said:
Portal 2 has never been directly confirmed by Valve. They have hinted at it before, but they've never said that they are actually making it.
Qoute from wikipedia(yes I know it isnt always the most reliable source,but still)
Swift stated that future Portal developments will depend on the community's reactions, saying, "We're still playing it by ear at this point, figuring out if we want to do multi-player next, or Portal 2, or release map packs."[13] On January 31, 2008, Lombardi confirmed that "more Portal" would be forthcoming, and promised that the additional content would not just be "more puzzles";[51] an interview with Kim Swift on February 21, 2008 revealed that a full sequel, Portal 2, will be coming.[52] Swift also stated that the team creating the Portal sequel is currently deciding upon the design they should implement and that a multiplayer Portal "from a technology standpoint is possible",[53] though Swift had previously stated they tried a Portal multiplayer variant, but admitted that "it's less fun than you'd think."[28]
On June 10, 2008, Kotaku reported that Valve was seeking voice actors for the character role of Aperture Science CEO Cave Johnson and suggested that the second game may be a prequel to the events in Portal.[54] However, a later update suggests it could be a sequel, with Johnson as another AI.
Now I'm not sure about you,but thats enough to convince me its confirmed.