http://www.gameinformer.com/News/Story/200608/N06.0825.1923.12789.htm
GI: You mentioned in the original presentation that Portal will take place in the Half-Life universe. Will that connection be made in Portal or Episode Two?
Lombardi: It happens while you’re playing through Portal. And you will play as a new character that we haven’t introduced yet that will have a role in Half-Life games in the future.