FYI, I am an attorney, and regularly work with air travel/disabilities issues. I don't know her credentials, but I respectfully disagree with some of what she says.
In particular, expirateshopgirl's summary of the ACAA requirements in post #35 is not quite accurate. The legal standard for identifying service animals for air travel is:
identification cards, other written documentation, presence of harnesses, tags, or the credible verbal assurances of a qualified individual with a disability using the animal.
14 C.F.R. 382.117(d) (emphasis added).
Note the word "credible." A passenger could not "tell airline personnel the animal alerts them to impending earthquake" (see post #31) and expect to make it on board the aircraft.
The Department of Transportation has specifically noted that if a passenger's assertions about a service animal are not credible and no documentation is provided, the airline is entitled to tell the passenger that "the animal cannot be carried in the cabin, because it does not meet the criteria for service animals." 73 Fed. Reg. 27660 (May 13, 2008).