A job listing spotted by Super Annuation seems to confirm what we already know: Microsoft is working on the Xbox 720.
The listing was pulled, but thanks to the magic of the internet, you can still read it. Here's the juicy part of the job listing:
"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 revolutionize audio for the next generation of Xbox."
We still have no solid information on the system, but the conventional wisdom holds that Microsoft could make a big announcement at E3 this year... seems like the perfect time for it.