The leaf type between biomes is on a block by block basis, and not per tree. We might have some funny crossbreeds going on between seasonal and pine forests (once they are added), but in this scene it looks nice. It will also be interesting to see how planted trees grow in biomes that do not generally have trees. Biomes are going to make terraforming more difficult, but exploration will definitely be more fun.
I'm hoping that biomes are not as frequent as this picture makes them appear though. I wouldn't want to walk for 5 minutes in any direction and cross every biome in the game. It would be nice to feel fully immersed in an individual biome, so that you would be able to look in every direction and really feel like you are in a desert or an ocean. If there are too many transitions, biomes will get boring and repetitive quickly, since you will constantly be seeing all of them, and it will make the world feel less epic.
I don't think so. If you look closely at the screenshot, some of the tree trunks look different too, and I have a feeling they all use the same block id to prevent block type inflation. Also, Notch said preexisting terrain from old maps would have the new biome grass/tree colors, which would not be possible if they were different blocks. It would make sense for the new 'useless' block to be something to give biomes more variety though.
I would imagine that the closeness of the biomes was deliberate in order show off the different ones without having to post a bunch of pictures. its much easier to see the difference here than if he were to just show us 3 seperate pictures, one thats a desert, one that has a lot of trees, and one that's icy.
I'm hoping that the forests will have taller trees. give it a less cramped feel than what a bunch of adjacent trees are like right now.