Tamiflu is the anti-viral drug that is currently regarded as being most effective against H5N1 influenza. Although it's efficacy is not easy to assess with out more data, responsible people (e.g. HSO Singapore) think that when used as treatment it can substantially reduce the likelihood of serious complications and that when used as a prophylaxis (say in hospitals or infected families) it can substantially reduce the infection rate.
People with a close personal relationship with a physician may still be able to get genuine Tamiflu if they take timely action. Once a pandemic is in place, Tamiflu will next to impossible to obtain except for people working within the health care system who are in actually in contact with infected patients. In some areas and some times even these people will not have access. There simply isn't enough to go around.
I plan to add some further comments that will summarize (1) the evidence for Tamiflu efficacy (2) the likely uses to which the stockpiles (federal, state, local) will be used. I'll also watch the traffic about the counterfeits; this is a situation where Gresham's law applies like gravity.
"I am pleased to announce the United States government has just deployed treatment courses of Tamiflu to a secure location in Asia. "
from Michael O. Leavitt's Statement to the World Health Organization (WHO) in Geneva May 22, 2006. The full formal action memo (link to be added) reads like a passage from a Tom Clancy novel.