Is it OK to tweet "movie" in a crowded firehouse?

Last week the G20 met in Pittsburgh. Amid the obligatory protests, one person was arrested for using Twitter to tell protesters that the cops were coming. George Washington University law professor Paul Butler gave an analysis on NPR. From a Field Notes perspective, the key quotes are:

[I]ntent or motive is key. So if the government can prove beyond a reasonable doubt that the idea was to help the protesters evade the police and to prevent the protesters' illegal activities from being discovered, then they've broken the law. But that's a lot that the government will have to prove and, you know, it may be difficult based on the evidence. [Earlier, Butler argues that telling protesters where the cops are could just as well be aimed at helping them conform to the law by making sure they know where protests are and are not permitted].

[T]he law is used to adapting to new technology, you know, for - there was a time when the telephone was new. And then there was another time when computers were new. And people used these new instruments for both legal activity and for political organizing and sometimes for illegal activity. And what law has to do is to figure out the difference.

From what I can make out, the gist is this: This particular case involving twitter touches on some very tricky issues of free speech, but the trickiness stems from the issue of free speech in general, not from the medium -- Twitter in this case. More specifically, the trickiness stems largely from the difficulty of proving intent, a difficulty not, so far, significantly affected by any known communication technology.