I know a lot of people are going to ask "why?" and stuff like that but I absolutely love butterflies and they bring games to life so easily. If you look at almost any game that involves the outdoors, you see butterflies or at least some other insects and I think Minecraft needs more life. The butteflies probably wouldn't have any drops, but they could land and fly away when players get near them. And like the aquatic update, you might be able to right-click them in a jar, take them home, put them in an enclosure, and they wouldn't despawn.
nice idea, a bit like bats but above ground. Nice to add a little more atmosphere to the world, especially when the super duper graphics pack lands. Perhaps localised to certain biomes where flowers tend to generate.
maybe even different colours in different biomes, or make some especially rare. Good fun collecting them in glass bottles maybe.
Minecraft has always needed something to make it seem more 'alive', and this just seems PERFECT. The butterflies don't really have a great impact on the game, but they just give a nice, real-world feel to it. They could spawn around flowers, and I like the suggestion of putting the mobs in a container. Later you can release them into a butterfly garden! I've always wanted one of those things.