This issue also seems to affect the current 1.7.9 version too. Passive mobs don't appear to spawn in Ice Mountains or Ice Plains biomes. Tried going 10,000 blocks in all directions around spawn (on a superflat snow world) and absolutely no sign of passive mobs. Also tried the new "randomTickSpeed" gamerule command in the snapshot 14w17a; increased that to 500 in an attempt to kind of force passive mobs to spawn in a lit area (with torches) but still no luck. Not entirely sure if this is a bug or an intended feature.

OS: Windows 7 Ultimate
Java: 7 Update 51