Turkey was engaged in aggressive action along its border with Syria during a particularly tense situation, and depending on whose account you believe, either flew into Syrian airspace or remained in international airspace just outside of it. While shooting down the plane was almost certainly an overreaction—the Assad government has said as much—it's hardly an "attack."
Ultimately, like the "high crimes and misdemeanor" threshold for impeachment set forth by the U.S. Constitution, it's a judgment call. In the former case, the House of Representatives makes the call; in the latter, it's the North Atlantic Council.
But it's virtually inconceivable that the NAC would deem this to be a qualifying "attack." First, Article 5 couches the response in terms of "the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations." An overly aggressive defensive action by Syria—especially a one-off—would not seem to qualify. While the Turkish pilot would certainly have been within his rights to use deadly force to protect himself, a retaliatory strike at this juncture by Turkey—much less its NATO allies—would be in violation of the UN Charter. Second, borrowing language from Article 51, Article 5 specifies the rationale for the use of force as "to restore and maintain the security of the North Atlantic area." Given that the incident is already contained—that is, not likely to be followed by any sort of follow-on action by Syria absent further provocation—said security already exists. Indeed, a NATO or Turkish response would make the area less, not more, secure.
A second misconception is that an attack under Article 5 will automatically be met by unified military action by all NATO states. Instead, a declaration by the NAC that Article 5 has been triggered is but a first step; decisions as to what response to take must follow. Not all attacks are equal. Even outside the politics of an alliance, states weigh incidents in terms of severity, the existing relationship with the attacking state, the international environment and the likely fallout effects of various response options.
Article 5 has been operative since the North Atlantic Treaty went into effect in 1949. It has been invoked and acted upon precisely once, following the Al Qaeda terrorist attack on the United States launched from Afghanistan. Even then, NATO’s response was cautious:
Article 5 has thus been invoked, but no determination has yet been made whether the attack against the United States was directed from abroad. If such a determination is made, each Ally will then consider what assistance it should provide. In practice, there will be consultations among the Allies. Any collective action by NATO will be decided by the North Atlantic Council. The United States can also carry out independent actions, consistent with its rights and obligations under the UN Charter.
Allies can provide any form of assistance they deem necessary to respond to the situation. This assistance is not necessarily military and depends on the material resources of each country. Each individual member determines how it will contribute and will consult with the other members, bearing in mind that the ultimate aim is to "to restore and maintain the security of the North Atlantic area."
. . . . If the conditions are met for the application of Article 5, NATO Allies will decide how to assist the United States. . . . This is an individual obligation on each Ally and each Ally is responsible for determining what it deems necessary in these particular circumstances.
Ultimately, of course, NATO decided to join the United States in its fight against Al Qaeda in Afghanistan. That some allies joined with more vigor and usefulness than others has been well documented. But that statement of September 12 outlines the nature of the Article 5 obligation nicely: the NAC may recommend action, but it's ultimately up to the individual allies to decide whether and how to respond.
Syria is No Libya