When Twitter launched in early 2007 I was largely dismissive of the service. In fact that was true until comparatively recently as well. Even as I recognized its value as a news dissemination tool (e.g., China Earthquake) and as a marketing tool (e.g., Dell's use of Twitter) and a potential local recommendations/social DA engine (e.g., MapQuest, Twitter + Summize) I was still not entirely "won" over.

Now I am. 

With the now familiar (on its way to iconic) image and subsequent news exposure of the US Airways plane in the Hudson River, shot and uploaded via iPhone by Janis Krums to the Internet and spread via TwitPic/Twitter, Twitter has entered the consciousness of the larger culture. 

Photo credit: Janis Krums

The popularity and "use cases" for Twitter have been building since its introduction, though a business model still awaits. And the company name had already become a verb of sorts ("Tweeting," "Twittering"); but this, in my view, is the breakthrough moment.