In Montana, most of the best brookie water is kept under wraps. I can tell you a dozen places where rookies could have a good day with small ones, but big brookies are hard to find.
The best place that I know of (that's not a secret) for big brook trout is Georgetown Lake. I've never fished it myself, but I've heard reports recently that they're back with a vengeance (it has suffered in the past 10 to years, apparently due to whirling disease).
Check out this video from Blackfoot River Outfitters (the brook trout action is from about 1:10 to 1:50):
The biggest brookie that Montana Fish Wildlife and Parks has surveyed there was about 20.5 inches in 2006. In 2007, about 60,000 4-inchers were stocked and in 2008,another 50,000 er so were added. By 2011, the population should be thinned out a touch, but their lengths should be getting up there. It might be a great year to investigate the brook trout bounty at Georgetown Lake.