I always thought that iOS required PVRTC textures, but I have my app loading a DXT5 texture. I stumbled on it when I started splitting my ATF file into individual formats.
All experiments done with TexturePacker. Observations:
1. Textures look great when using combined ATF (DXT, ETC, PVRTC)
2. Textures look terrible when using only PVRTC ATF
3. Textures work and look great when using only DXT5 ATF
Maybe this is old news but I wasn't able to find anything about it.