On his blog, James Fallows gives several examples of Mexican citizens being quarantined in Beijing solely on the basis on their nationality. He says the reports are “based on first-hand reports from people I trust.” From the first example:
A family of tourists — two parents; a son age 8; and daughters ages 6 and 4 — were staying in a five-star Beijing hotel. Like all foreigners in China, they had presented their passports for inspection on arrival. Their passports were from Mexico. At 4 am last night they heard a pounding on the door. Public-security officials asked them to come to the hospital for a few quick tests. In fact they were taken to a hospital and not allowed to leave. They received no drugs or treatment of any sort and were placed in a room where the beds and sheets still bore the marks of the previous ill and bleeding patients. They managed to contact Mexican officials by phone — which was the first the Mexican government had heard of their situation.
See also “Netizens Angry Shanghai Allowed Swine Flu To Enter China” from chinaSMACK.