Looks as though the speculation that Half-Life 2: Episode 3 would be Valve's "big surprise" come E3 2010 was shot down in fiery flames. Although it seems rather odd that Valve head honcho Gabe Newell would shoot down candidates for the mystery reveal, apparently he confirmed Episode 3's exclusion in a recent private message, simply saying "it isn't Ep 3." Bummer.
Newell also confirmed that a recent teaser image--depicting the Half-Life symbol with the words "Freeman" and "June 14th" centered underneath--was a fake. The history of the document also shows a lot of tampering although the edits appear to have taken place on June 4. Clearly someone was really hoping for the long-delayed, third installment, and wanted to spread the love.
So what exactly will be Valve's big surprise? The company recently hinted that the mystery reveal wouldn't interest Xbox 360 and PlayStation 3 gamers, indicating that PC gamers will receive some sort of high-profile treat come June 14. That's a nice surprise in itself given that many PC developers are slowly shifting over to the dark side (namely heavy console support).