I visited Illinois Peak September 13, 2013 and the first mile of the trail is no longer derivable to highway vehicles. There is a kelly hump and boulders preventing travel to all but dirt bikes at the Freezout Pass Trailhead. Exceptional hike though- includes a stroll through one of Montana's only mountain hemlock forests!
Illinois Peak's summit and more than 60% of the mountain are in Montana, NOT Idaho. I've been there when a certified surveyor took new GPS measurements and it's a Montana mountain by default.