The first teaser posted by @KitKat reads "Everybody dance now!" and is a reference to a song named Gonna Make You Sweat performed the C+C Music Factory band. Interestingly, the original name of the band is “The 28th Street Crew.”
And then comes today’s teaser with the “This is it” reference. “This is it” is a name of a movie dedicated to Michael Jackson and - interestingly enough - it was released on October 28th. Both teasers share this reference to October 28th, and this does not seem like a coincidence.
Now you might think we are reading too much into this, but an earlier hint about October 18th launch of the Nexus 5 and KitKat was just not realistic (even Google would not risk inviting the press and giving them merely a 2-day warning).
Moreover, this new date falls perfectly in line with last year’s Google announcement of the Nexus 4 that happened on October 29th, 2012. We'd thus bet on October 28th being the real date of the Nexus 5 and Android 4.4 KitKat announcement.