In this post I will keep bits of info about the next update for minecraft.It will be updated very often.
It will be helpful to post information that you find, in this thread.
- Snow and rain will return and will affect gameplay. 
- Thunder and lighting are added.
- Monsters spawning in daytime, outside during Thunder storms. 
- Achievements and Stats are planned to be released in this update.
- Saplings for specific trees.  
- Powered rails (minecart boosters) added.
- Pigs turn in Zombie Pigman when hit by lightning.
- Rail Detector. 
- Allow-Flight parameter for SMP (anti-cheat feature) 
- A tree now drops less saplings. 
- Occlusion culling added to Video Settings. 
- More harvestable items ? 
- Jump attack ?  
- Bow Aiming ? 
- Automated Harvesting with water droping seeds? 
- Fence Gates ? 
Screenshot showing the border of two biomes.
Video of Snow,Rain and Achievements.
- Water Flow back to normal. 
- Piston mod won't make it to Minecraft 1.5 
- Modding API won't be in 1.5.