Is it considered a loss if you logout or close the game during the countdown?

You search for a game on Battle.net, the countdown starts and at the same moment your telephone also starts ringing. Suppose you want to take that call.

Is it considered a loss if you logout or close the game before the actual game begins?

According to the patch notes released today:

A player who logs out of the game during a match countdown now receives a point loss. The other player now receives a message that the opposing player has canceled the queue and neither receives a point bonus nor penalty.

So yes, you will take the loss by closing out/quitting the game.

Source: http://us.battle.net/sc2/en/forum/topic/5271506462?page=1

Yeah, it ends up in a loss. Here's Destiny raging about it.

This answer is now (with Patch 1.5) outdated.

If you manage to logout in time you will get a tie, as shown in the picture below. I guess if your computer freezes or you try to Alt F4 you might get a loss. The following reasons might apply:

• Killing a process with Alt F4 is not instant. Every process is given some time to shut down properly. As you only have 3 seconds before the game starts, this might not be enough. Especially on slow or otherwise occupied computers.
• Exiting Starcraft with Alt F4 might not send a proper logout signal to the sever. Which means you are considered online for a short timeout period.