That's 100 percent confirmation that Portal 2 will be, in some capacity, at E3 2010. Recently they just announced it will be out 2011 sometime.
You want evidence? Here's your stinkin' evidence!
“To ask questions about how close we all came to dying, or to ask futile questions about the previously announced E3 ***PORTAL-2-THEMED-FOR-GOD’S SAKE*** surprise…” was attached to the note detailing Portal 2's 2011 postponement.
Let the hype around Valve's E3 'surprise' settle die finally...
Source: VG247