While not a direct confirmation, NeoGAF noted that during the latest Major Nelson podcast episode, Aaron Greenberg, chief of staff for interactive entertainment at Microsoft, seemed to imply that whatever is shown during the Xbox reveal will be out this holiday.
The assumption, of course, being that they're going to show the next Xbox during the event, and therefore he said that the next Xbox will be out this holiday. To quote:
We're really going to tell one story across two events. So we're going to start on the 21st and really that's about revealing the next Xbox platform, really our vision for the future of games, the future of entertainment. We definitely have a lot of surprises planned....at E3, I mean, if you're a gamer, it's going to be tonnes of exclusives, world premieres, and obviously even beyond that we have a lot to share between then and this holiday.
Emphasis mine. We've reached out to Microsoft for confirmation about this, and will update this post if we hear back. Otherwise, here is everything we know about the next Xbox.