For all of those looking for a Valve announcement at E3 for either Half-Life 3 or the rumored Steam Box, it looks like you are not going to get one. Doug Lombardi of Valve Software confirmed that Valve will not be exhibiting at this years E3 event which is to be held June 11-13th. Doug left things pretty simple as you can see below, no really elaborating as to why Valve will not be exhibiting, but we have a few ideas.
Valve had been reportedly working with Xi3 on a Steam Box console. Since CES we have heard that the relationship between Valve and Xi3 has fallen apart. We are not really sure what is going on between Valve and Xi3, but it looks like there will not be a Steam Box announcement at E3. That suggests that if there is in fact a Steam Box in the works it will not be ready by then and we will most likely not see it in 2013.
We have not seen a new Half-Life title since Episode 2 was released in 2007. Some thought we might see Half-Life 3 announced at E3, but that is unlikely to happen either.
Valve may still have a presence at E3, but do not expect any announcements.