A Microsoft job listing that's since been pulled, via superannuation, has explicitly said it's working on the "next generation of Xbox", whilst a posting for Forza Motorsport developer Turn 10 has said it's working on its "next Generation of games".
The MS listing for a software development engineer, which has since been pulled but captured by Google Catche, explicitly says it's for the next generation of Xbox. As so:
"The ISS Console Multimedia Team, responsible for designing and developing the next generation console, is looking for a highly motivated and skilled audio developer. If you love gaming, game audio and music, then join us as an integral part of the team tasked to revolutionise audio for the next generation of Xbox."
Various rumours have pointed towards Microsoft working on a next-gen Xbox, with the most recent report pegging it not to have a disc-drive.
Turn 10 is also hiring for next-gen, it seems. The Forza Motorsport studio has posted a listing for a cinematic art director, stating it's hiring for "our next Generation of games".