rst off: I may not be a medical expert, however, my parents are. Secondly, here's a source, the Centre for Disease Control (America):
CDC H1N1 Flu | Interim Recommendations for Facemask and Respirator Use to Reduce Novel Influenza A (H1N1) Virus Transmission
The most effective use of face masks is probably to prevent already sick people from coughing the particles out, but the masks won't stop the spray completely and are altogether useless once they become saturated (as I said, after about 30 minutes of use, from both the air and one's own breathing). They are also only to be used once, yet many people ignore this.
So what am I saying? If everybody wore face masks 24/7, replaced theirs every 30 minutes (but did not remove it and place it back on at all), AND avoided all physical contact (including with things touched by potentially infected people, such as keyboards, mice, taps, handrails), and washed their hands up to their elbows for 30 seconds (the proper method), then the rate of transmission
may drop significantly (we don't know whether face masks would even do anything if they were used!). Otherwise, snowball's chance in hell.