Sneezing
According to Peter Lorie, in Superstitions, someone saying "Bless you" in response to another person sneezing started back around 150 AD. Tiberius Caesar supposedly say "Bless you' whenever he heard someone sneeze - or "Bless you, bless you my dear" if the person sneezed a 2nd or 3rd time.
So the blessing of someone when they sneezed started not out of etiquette thoughts but due to a serious illness back in that time period that many early Romans died from. One of the symptoms of the illness was, as you may have guessed, sneezing. The reason why Tiberius would say "Bless you" was due to the belief that the more blessings offered to the sufferer may help lessen the chance of death.
Old Wives Tales
At one time people believed a man's soul could be inadvertently thrust from his body by an explosive sneeze, thus "Bless you!" was a protective oath uttered to safeguard the temporarily expelled and vulnerable soul from being snatched up by Satan (who was always lurking nearby). The purpose of the oath was to cast a temporary shield over the flung-out soul which would protect it just long enough to regain the protection of the corporeal body.
John Mark Ministries
A couple of quick Googles above...
Either way, the sneeze aspect is interesting. What is the illness that Bush is trying to protect the US against? Or - was the soul of the US recently expelled from it's body?