Sadly, no official word just yet. The countdown does include logos for Valve Steam and the Source Engine, but that's about it. The countdown is planned to end at August 21(US).
I did get excite when I first read the news and saw the countdown with my own two eyes, but it then struck me - Valve announces Left 4 Dead 3 through a countdown!? The Left4Dead franchise is considered a huge success for Valve. Heck, the original game and the second game is still being played as if it just got released. Now with that in mind, I'd expect them to make a big reveal in an event of some sort like E3 to maximize awareness to your market. So, it's really hard for me to go all out excited.
Valve has not officially confirmed that this is legit, but only time will tell. I'm sure the company is well aware of this countdown as I write this, so I already see myself updating this post either later or in a couple of days.
[Source: IGN Asia]