According to “third-party publishing sources” close to online publication MCV, Microsoft will unveil the successor to the Xbox 360 (aka the Xbox 720/Xbox Ten/Xbox Loop and/or Xbox Next) at the E3 annual video game conference and show later this year in June. MCV says that details on the Playstation 4 will also be revealed at the show.
Apparently Microsoft has signalled in advance to its gaming partners that the Xbox 720 will make an appearance then. A PS4 reveal at the same time also makes sense, given that there is no way Sony will want to be left behind in the next gen console race, like they were with the Playstation 3.
As always, take this rumor with a grain of salt, just like the dozens of others that have proliferated the internet over the past few months (and most of which we have dutifully reported on here).