There is one big difference though, they have government plans and insurance is supplemental, like the UK has with it's options of private or state health care as a choice. The companies have to be there in China for a couple years though before they can really jump in to the main swing of things though via government rules and regulations.
Clearly, a lot of the growth will come from China's booming middle class, but there's more to the market than just the increasing number of people who can afford coverage. Expatriates at multinational companies that have expanded into China are also demanding the same sort of health insurance they have back home. Health insurers probably don't have to maintain a base in China to sell health insurance to multinational companies, but it certainly should help those potential corporate clients determine health-care costs.
Cigna is clearly one of the early movers, which should help it grab market share, but I'd guess that U.S. firms' choice of Chinese partners will play an equally important role in establishing the eventual market leader.