Adhesive
.008 of an inch of RTV silicone
According to this dated but very interesting material, the tiles are held on by silicone. I know some automotive silicones used for sealing are rated to 700º F.
I had no luck in searching Dow's Website for more info.
(I saw some thermo characteristics of the shuttle tile demonstrated at KSC. The speaker heated a tile sample until it glowed red-hot then removed the torch and immediately place his hand on the hot spot. The surface was cool.)
Amazing