Daniel Knight (writer/director) has been trying to make this for 8 years now
He founded Snowgum Films to attempt the Troll Bridge adaptation, and what you're seeing now is actually the /second/ variant.
Early on, in the dark days of the internet, a handful of people donated some money for the promise of a DVD. Back then, when Troll Bridge began production, it would have been the first live action Discworld adaptation - this was after the Cosgrove-Hall animated versions of Soul Music and Wyrd Sisters, but before anything at all had been done by The Mob/Sky One.
In the end, timing and lack-of-financing got in the way, and there was a fan made version of Lords and Ladies filmed in Germany, and the person who'd volunteered to do the troll CGI found his day job took up too much time, etc etc. So it got put on the back burner, and Daniel got on with making other more affordable films - including "Run, Rincewind, Run" for Nullus Anxietas, the first Australian Discworld Convention in 2007.
At NA2 (2009), we were treated with some early test footage of the "prequel" part featuring Young Cohen battling two entire armies. This was a completely rewritten and re-shot version of the prequel part - the original has Cohen defeating an evil wizard and saving a young vestal virgin. But armies are more epic, even if the special effects weren't complete, and each army was basically the same ten guys in different costumes.
Fast forward to Nullus Anxietas III in April 2011, and Daniel launches the Kickstarter campaign for Troll Bridge - if they can raise $45,000 by July, the film will definitely get made. If not, he will finally give up on the project after 8 long years.
By the time the closing of the Kickstarter rolls around, it's hit $82,000. Almost twice what they'd asked for.
So they flew to New Zealand and shot some of the footage there. There's still a fair bit more to shoot (pretty much all of the main part of the story on the bridge), and a lot of post production work after that, but now that it's funded, it's definitely going to get made.
I caught up with Daniel and his girlfriend in Melbourne about a month ago, and we chatted about their time in New Zealand and what the shoot was like - some good stories there!
Daniel will be one of our visiting lecturers (guest of honour) at the Unseen University Convivium next July, and he did mutter under his breath that it's possible we might be able to see some of the bridge footage by then. I suspect this means the Troll special effects won't be complete, but it'll still be a nice treat to see anything new out of this!
By the way, even though the Kickstarter has closed, if you would still like to donate some money in order to secure yourself one of the available rewards (including DVDs, Blu-Rays, Posters etc), then you can read about it at http://www.snowgumfilms.com/trollbridge/backing.html
Hope to see some of you at Unseen University Convivium 2012!