ValveTime.net forum member and Valve Software head Gabe Newell.
In the purported exchange, which Valve declined to confirm, Newell told the forum member, "We are not announcing anything at E3. Really." The email goes further than that, adding, "We are not announcing Half-Life 3 or Portal 3 or Left 4 Dead 3. We are going to be showing stuff everyone already knows about (CS:Go [Counterstrike: Global Offensive], Dota 2, 10' UI [Steam 'Big Picture Mode'])."
So, while it looks like Valve is attending E3, don't get your hopes too high for any software or hardware announcements. The other possibility, of course, is that we're being lied to (it wouldn't be the first such instance before E3). We're giving Valve the benefit of the doubt.