Ian - check out the Del McCoury band. They are one of the hottest "traditionalist" bluegrass bands working at this time.

I haven't seen "Down from the Mountain" so I'm not sure who is in the film, but as far as classic bluegrass there's Bill Monroe (the "father" of BG), the Stanley Brothers, and Flatt & Scruggs. For the New Trads, there's Gillian Welch, Alison Krause & Union Station, Nickel Creek, Steve Earle, and Alison Brown. You might also look for some Ricky Skaggs albums (beware, he went through a Nashville country-pop phase but is playing BG again). And of course The Queen, Emmylou Harris.

Also of note this year is the 30th anniversary of the Nitty Gritty Dirt Bands groundbreaking collaboration album with dozens of the greats of pre-pop country and BG, "Will The Circle Be Unbroken", which is being reissued. A must-have, timeless classic!

Hope this will get you started; if you have more specific questions, post them here; there are others more qualified than I to answer them (I'm mainly a Celtic guy these days).

Cheers,
Michael