I'm with Gonzo; there's no reason at all that non-native speakers should not be as good at teaching English as native speakers, and they even have some advantages.
Also, there are some fairly odd dialects among native E speakers. My part of Canada has people who say things like "I seen Fred yesterday." and I would not say they would be qualified as teachers unless they've also learned standard English as a second dialect. There are equally odd things in other E-speaking countries.
People from Holland or Scandinavia rather often have excellent English, I think partly because their native languages are related to E and partly because those countries have good schools with required English courses. An argument they might use is that both the really massive definitive grammars of E have had a Scandinavian author. Otto Jespersen, a Dane who I'm told never lost his rather thick accent, did one about 1900. That was superseded by Quirck, Greenbaum, Leech and Svartvik in the 1970s. With a name like Jan Svartvik and a post at Oslo University, I feel safe assuming one of those is Scandinavian.
That said, I'll quote text I mostly wrote from the Wikitravel China article:
http://wikitravel.org/en/China#WorkThere are a fairly strong preferences for native English speakers and for citizens of major English-speaking countries. Job ads routinely include a list of acceptable passports; UK, US, Canada, Australia, and New Zealand are on every list, Ireland and South Africa on most. Some schools will not even read the rest of your resume if you do not have one of those passports. Various prejudices may also come into play; overseas Chinese (even with perfect English), Filipinos, Indians, Malaysians, American Blacks, and especially Africans all report some difficulties finding jobs, or getting lower offers. Members of all those groups are happily employed in other schools, and many are well-paid, but getting a job is easier for people who fit a stereotype — Caucasians especially Americans or British. Some schools want blue-eyed blondes, because they hope that will help their marketing. Accent can also be an issue; Chinese people generally hope to acquire American accents, so a really thick Scots or Aussie accent will bother some employers, for example.