The existence of Hawking radiation has never been observed. In June 2008, NASA launched the GLAST satellite, which will search for the terminal gamma-ray flashes expected from evaporating primordial black holes.
Not exactly proven, but neither are black holes. Although there is observational evidence for them, unlike Hawking radiation. The problem is that with any black hole we may discover, the Hawking radiation would be so feeble that even the cosmic microwave background would dwarf it by a huge factor. But nevertheless, the first two reasons in my original post serves to make this particular fail-safe just a redundancy.