Hey, Twitter guys, quit teasing us!

Twitter CEO Evan Williams gives a very open hint.

(Credit: Twitter)

Something's afoot at Twitter. Well, something's always afoot at Twitter, but this time the founders are leaving everyone hanging by Twittering about it.

CEO and co-founder Evan Williams posted to his Twitter account on Thursday morning: "Tomorrow just became a very big day. (Sorry for the teaser--more later.)" Ooh! Fascinating!

So what's up with his fellow co-founder, Biz Stone? He's in Boston, which happens to be where one of Twitter's big investors, Spark Capital, is headquartered. Spark led Twitter's $15 million Series B funding round last year.

"It's almost midnight in Boston--did I really agree to get up before 6am?" Stone posted to Twitter late on Wednesday night. "Tomorrow is going to be a long and interesting day!"

So will there be a big Friday announcement from Twitter? Who knows! Since Twitter raised additional funding from Benchmark and Institutional Venture Partners just two months ago, it's probably not more venture capital. There's a chance the announcement will turn out to be, well, nothing.

But seriously, 140 characters can be such a darned tease! Leave comments with your best guesses.

UPDATE at 11:02 a.m. PT: If this is to be believed, looks like Oprah Winfrey is starting to use Twitter on Friday, and that's the big news. Oh, well.