So when's Cassiopeia getting nerfed?

Never, because she doesn't need them.

http://24.media.tumblr.com/ffae5844e27f4c850d828587a24f0e7c/tumblr_n2k0kpqeEx1rk7zoeo1_400.gif

Do you want to elaborate on just what makes her so "OP"?

I believe her ridiculous power comes from the same source that can make several other mages pretty unstoppable in a game...Rylai's having a proc EVERY time a DoT ticks (or her .5 sec cooldown spell). Combining the permaslow with the pool that prevents non-ranged champs from getting to her might make her the safest mage in the game right now against non-ranged champs (typically assassins).

I think that riot needs to put a reasonable length cooldown on Rylais to prevent the perma slow that it gives so many mages right now. Riot says that they want interactions in the game to be "man I could have played that fight better" over "There was nothing I could do" but as a non-ranged AD character against her after she gets Rylai's, it's basically always the second one.

Cassiopeia is a very hard to play late game mage

1. she has very high mana costs making her a "use then refill" champion not good at lane sustain unless you're just good with her
2. she doesn't have much protection besides her ult which is very unreliable
3. 3 of her abilities are skillshots, making her harder to play than most champions

After get justified nerfs.

I.e. never.