The U.S. Embassy in Mexico City has suspended processing of the approximately 5100 non-immigrant visa appointments scheduled Monday, April 27 through Wednesday, April 29. In the US, All USCIS offices remain open for business. Applicants should plan to attend all previously scheduled appointments, interviews and ceremonies unless they are feeling ill. It seems that re scheduling due to illness is a perfectly acceptable reason.
There are no reports of problems to enter the US from Mexico, no refusals due to Swine Flu issues as of now. But this may well change as the issue develops.
Follow the WHO updates here