That's right! Watch my sim Leaf come across an interesting revelation: http://www.youtube.com/watch?v=9CIfyyKbRDQ
You need the Ambitions expansion pack to do this. Buy the fog emitter under the misc. tab using the buydebug cheat and shift ctrl click on it and there's a place to type in effect names. Type in snowfallinglightday to get some snow! You have to place multiple emitters to get snow all over the lot. It can snow inside however. This does not have any effect on gameplay, it's an effect and nothing more at this point. It can also set up to be triggered through the hidden switches from World Adventures as shown in my video.
Weather just might be in EP3 folks. Seems like we've got confirmation that it's at least coming eventually!
Edit: Okay...looks like it's not EP3 then, as it's confirmed to be Late Night (Nightlife with Superstar and a bit of Apartment Life mixed in).
Special thanks goes to Goggalor from Mod The Sims for sharing how to do this! Original thread: http://linna.modthesims.info/showthread.php?t=405544
I'm going to go do some more experiments with this, the possibilities here are huge!