I just had an amazing experience.
I was working on the redstone circuitry for one of my stations when it started to rain for the first time. The sun was about to set, but I didn't want to miss out on my first opportunity to experience rain by cowering from the monsters, so I set the game to peaceful and went for a walk.
By the time I reached my main base, the sun was starting to come up and I decided to see what snow was like, so I went into the Nether and exited at my snow biome. I walked around for a bit enjoying the calm when I realised that snow was also falling outside my snow biome, in land that had been generated before the Halloween update. So I entered that area and walked around as snow slowly accumulated around me in patches. I suddenly realised that I was witnessing a unique event, the birth of a snow biome, and that I would never experience this again. This can only happen in cold areas generated pre-biomes and this is the only such region I have.
Anyway, here's the money shot:
As you can see, the process isn't complete, the snow stopped after 6 or 7 minutes and it's very patchy in the north and east. Also, ice didn't form in the new region or in areas where I had broken it in the past. It seems to be connecting the small snowy region in the north with the large snowy region in the south, which means that I'll have a massive snow biome once the process is complete.