Depending on the release time frame, we might not see an unveil until E3.
E3 sure does look exciting next year, if not revealed until then, publicly of course.
My impression is that E3 is nowhere near as important as before, these companies need their flagship products to have their own show and prime time.
It's not desirable to unveil PS4 and Xbox next at the same time, for the first time, at E3, with the press splitting their coverage and attention, with the two products getting heavily compared and shitstorms happening whenever one has a better feature than the other.
Just to make an example the 360 presentation in 2005 was completely dwarfed by Sony showing target renders and tech demos at full specs. The poor J Allard was basically getting asked during interviews if their hardware was a turd...the day of the presentation....
That is something that they will be careful not to repeat, it's crucial to set the right mindshare since the beginning, you can't do that with a potential time bomb sitting in the nearby press room.
So I think that both PS4 and Xbox will be unveiled before E3 at dedicated events.