4chan's /v/ decided to visit Gabe on his birthday and sometime during their chat Gabe was asked about if a new engine was in development to which he responded "Yeah!". You can't really hear his voice too well in the video but that's what the subtitles are saying. Hopefully this is true, I would love to see a new engine from Valve... And with it hopefully HL3.
Gabe Newell Confirms That A Next-Gen Valve Game Engine Is In Development*|*LambdaGeneration