I'd put my money on it being a behind-the-scenes update and not grapical, which doesn't bother me one little bit.
We can only hope it doesn't break the mods, you'd have A LOT of people raging.
That is, unless they release it as a seperate game.

EDIT: Just noticed there was a video that was posted...
Looks like it's not a cosmetic update, no new graphics, but I really like the idea of achievements finally coming to CSS