I've definitely seen fake Arc'teryx stuff in the "copy mall" in Shenzhen. 5-6 stories packed with tiny shops, all selling fake stuff. (see http://www.youtube.com/watch?v=G7wS-wPrw6o
(it's mostly hidden, but not very well. Some of the name-brand stuff the cops "watch out for" there is hidden in the ceiling tiles, which pretty much have a network of interconnected stashes for various same-owner stores). It's supposedly a "legal mall", but it's well known as the place where you can get pretty much anything knockoff. Barely anything is actually legit, at the very least it's "Tony Hilfinger" or "Rolox" if not a direct knockoff. Most of the stores sell watches, copied DVDs, and/or purses, but at least 5 or 6 booths sell knockoff outerwear.
Kinda a fun place to go, if just for the hilarity of China's "strict copyright enforcement". Did I mention this place is literally across the street from the border with Hong Kong, with plenty of immigration/customs officers around? Yeah...
Anyway, yes, there's definitely copied arc'teryx stuff out there. Not uncommonly the factories that actually produce the real items will just order extra material and claim it as scrap or rejects, and either keep running jackets (with much lower quality control) or alter the design to something easy to make with the same fabric and real tags and make more to sell online/in stores for some quick extra cash. The labor doesn't cost much, it's setup and fabric that costs a lot - so the cost of running more jackets through is tiny for the factory. I bought a "north face" softshell there that I'm sure is actually gore windstopper - and the tags, embroidering, and labels were all 100% perfect. It's certainly not ski-gear quality though, and I'd never rely on it for more than walking around town. Was just way cheaper ($32 after a good bit of haggling if I remember correctly?) and better quality than any other softshells they had there!
An easy way to spot obvious fakes is to check the tags and labels. Very often there will be misspellings, incorrect translations, bad grammar, or just flat out gibberish. Check the tag edges for clean cuts and whatnot too, and for paper quality and print quality.