Update (5:22PM)
Looks like China is putting a new spin on the arms sales. Apparently they're going to punish the American companies who are producing the arms being sold to Taiwan. No further details are being provided on the new punishment, but contracts could be modified or canceled. It seems the company that has the most to lose is United Technologies, as they have quite a bit of commercial (read non-military) business in China and are building the Black Hawk helicopters. Boeing probably won't be impacted too much, as they're only delivering a few Harpoon missiles. Should be interesting to see what happens.
Part of me wonders how much of this is for show and how much is real. Beijing has to realize that Taiwan being within the orbit of the U.S. is not the worst scenario (it could declare independence). Granted American involvement in the Taiwan issue has some overtones of imperial intervention, but it's far less than what the British, Russians, or Japanese did to China in the late 19th/early 20th centuries. Moreover, American involvement restrains Taiwan from doing anything that would force China's hand in attacking the island (independence or developing nuclear weapons). Not saying American intervention is right or wrong, but things could be far worse.