From Anime News Network:
The October issue of Neo magazine (A UK Anime magazine) includes an interview with Studio Madhouse's founder, Masao Maruyama. In it he says the studio is working on a Trigun Movie that should be released in a "couple of years". The November issue of Anime Insider also confirms this news.
Considering that it's been like 7 years since Trigun first came out, this is more than a little bit of a surprise. From what I've heard, Trigun wasn't all that popular in Japan so the question becomes, is this something initiated by the the show's popularity in the US, or did Madhouse Studio just feel like re-visiting the characters. The other question would be whether or not this is going to be an original story or a re-cap of the series. There's plenty of material in the manga that wasn't covered in the original anime, so hopefully they'll use that material. We'll just have to wait and see.