I have been stopped at PP by an imigrations/customs officer equiped with a sat. phone if he needed to verify anything.  Thankfully, my group did have all the correct paperwork and the process was carried out in 5 minutes during our wait at the ranger station.  That was in 2000, so had no post 9/11 implications.

Of course I've also been stopped at every single portage between Birch and Knife by alternating USFS and Quetico rangers, on the same day.  It was cool though, because at least the rangers have some good stories and information. The immigration officer seemed out of place, like he wanted to strip the bow plates off our canoes.  ;D