Although it may seem far away, Mojang has gone straight to developing 1.4 - even though 1.3 has just come out.
There are already updates to villager trading. The way it will work will allow you to keep some good trades, assuming you use new ones to renew them.
The command block. This block isn't craftable yet, but it's id is apparently 137. I'd assume by some sort of trigger, possibly hitting it, it would automatically make you use a command. This could be one amazingly useful tool for maps. What are your thoughts?
There is also going to be a suprise block. What do you think it will be? A redstone type, just another building material, who knows?
There is also supposed to be a new kind of zombie - a baby zombie! From what I've been told by other players, they are made when zombies eat baby villagers (and yes, apparently villagers can get infected when killed).
Zombies and skeletons can now have gear equipped, depending on difficulty. From a screenshot shown, they had enchanted diamond armor. This could become an easy way to tell what they might drop...
So, what do you expect 1.4 to bring?
(Info mainly from minecraftforum)