Actually, I've heard the down-low on all of these-
I've personally addicted several friends to the series, before breaking it to them that it got cancelled mid-season-1.
The movie is supposed to take place six months after the last episode that was filmed (I think they're using the DVD as the base, rather than the series, because there were some episodes that didn't even get aired on TV)
There's a new investigator put on Simon and River's case, and he's VERY good at his job. Good enough that he makes life uncomfortable in the extreme for them.
There are supposed to be Reavers, too... actual shown Reavers. I'm kinda excited about that.
That, and having Jane say funny things.
I love the writing of that show.
The trailer should be around a few weeks before Hitchhikers Guide to the Galaxy, online, and then on celluoid.
-Kid Vid (movie nerd hookup)