I love this place and so feel compelled
to add some less-factual, subjective
"information" that might in turn compel Northwesterners
to visit this beautiful natural preserve.
The widerness area is bordered by Canada on the north, but the trails do not end there. On the other side of the border is Manning Provincial Park in British Columbia, also a decent place to take a hike. This must be the easiest land border crossing in the United States. The border there is actually quite an interesting sight- it consists of a swath cut directly through the forest, about 20 meters wide if I recall. Crossing the border on the Crest Trail, there is a small metal monument several feet tall off to one side of the trail. If you ever pass by there, lift the top off. There are wonderful things inside-- leave one of your own behind (but otherwise Leave No Trace).
Personally I find the west side of the Cascade Crest up here to be better backpacking, simply because of the incredible vistas of gnarled peaks available from the top of any ridge. But the eastern walk in from Iron Gate has its charms as well, and affords interesting views east into the Okanogan and Methow valleys.
I've walked thru the Pasayten from North to South on the Pacific Crest Trail, and from East to West on the Pacific Northwest Trail which there is often called the Boundary Trail (it follows the Canadian border much of the way, hence the name). Both times we exited at Ross Lake, which is an extremely long lake which stretches its northernmost finger into Canada. Boats are available to pick up hikers at places like Lightning Creek and ferry them back to Ross Lake Resort. If you get a nice "ferryman" and you prod him, he might take you back into one of the many gorges off the East shore of the lake. There's one in particular that ends in a spectacular waterfall.
Anyway, I'm rambling. If you're into backpacking and you're in the Washington/British Columbia area, you should go there at least once. It's preserved as a wilderness area so that you can appreciate it; take advantage.