Hi Scott;
"I've hooked it up on the breadboard as it's shown in the data sheet but it's giving me 7.5V output. I am only supplying it was 12V. Apparently it has a maximum input of 26V? How is that possible given it's meant to have a maximum output of 5V? I tried a couple of minor variations on the hookup but same result."
Well that is a very strange and unusual result. The " I tried a couple of minor variations on the hookup but same result." has me a little concerned, as there is only one correct way to wire it up. A Yoda would say "no variations, just correct way" ;).
Do you have any kind of load wired to the regulator like say a 100 ohm resistor or are you reading it with the output pin floating? Are you using any recommended input and output capacitors that the data sheet might recommend. I've not used that series of regulators before but have used dozens and dozens of the 78XX and 79XX series and never had a bad one. They are pretty much industructable because of their automatic foldback/shutdown features on over current and over temperature.
I guess you can only review the hook up once again. Source voltage is 12vDC (not AC), 12v wired to corrent input pin, ground pin of regulator wired to ground of 12v external source, measured output between correct output pin and ground, install capacitors to insure regulator is not oscillating, check voltage with output wired to a resistor load. Then check again. 
Lefty