Fallows has the scoop:
You can understand why China is nervous, given its dense urban populations and its experience with SARS. You can understand quarantines based on recent presence in a diseased area or possible exposure to diseased people. You can comprehend why direct flights between Mexico and China have for now been called off.
But there is no decent reason for quarantine and detention based solely on nationality. To the best of my information, this blanket quarantine of Mexican citizens is not being applied anyplace else on earth. Let's hope this is a panicky mistake by Chinese and Beijing-area officials and will soon be reversed. It is also worth recognizing the overall aplomb and openness that the Mexican government has been showing in handling the flu outbreak.