A Twitter pitch isn't copyrighted because no pitch in any medium can be copyrighted. You can't copyright ideas. You mentioned "Rome, Sweet Rome" -- here's the pitch (126 characters long so it could have been pitched on Twitter): "a unit of current U.S. Marines are suddenly transported back to ancient Rome and forced to do battle with the Roman legions." There is NOTHING in U.S. law that can legally bar YOU from writing a movie based on the idea "a unit of current U.S. Marines are suddenly transported back to ancient Rome and forced to do battle with the Roman legions." Feel free. There's nothing to stop a studio from buying your script, producing it, and releasing the movie, either. Here's some more ideas you can -- perfectly legally -- base your next screenplay on: 1. Four guys go to Las Vegas for a bachelor party. Three of them wake up in the morning hungover and don't know where the groom is. 2. A lawyer's son makes a birthday wish that his father won't be able to tell a lie for 24 hours. 3. A TV weatherman goes to Pennsylvania to report on the groundhog and finds himself reliving February 2nd over and over. 4. An asteroid is on a collision course with earth, so NASA recruits a team of expert miners to go into space and blow it up. I could go on, but I imagine the point is clear. Ideas cannot be copyrighted.

This is exactly right, with one exception: short phrases can be trademarked, for instance Disney's "Where dreams come true". Doesn't stop anyone from taking the phrase/pitch and writing a new script, but it does prevent anyone from using the phrase, for example for marketing/promotion. Also, the bar for plagiarism cases is insanely high. AFAIK, in all these years, only Art Buchwald ever won his case.

There were a couple of reasons why I asked. First, someone might have the idea of doing a Twitter pitch contest where people post their loglines as tweets. If it's something you just came up and you don't have a full registered synopsis or script, that would have made it harder for someone to steal the concept. Second, it may happen that a tweet of mine can become the basis for a script. Let's say I write today "Hey, they never made a Hercules vs zombies movie, did they?" and while I start working on the script, Universal announces a Hercules vs Zombies movie, may I use my tweet at least to prove that I didn't steal their idea?

Hmm... maybe Twitching™ is the wave of the future.