Can't people still find a way to mod the game. Didn't that happen to battlefield 1942...no sdk yet a few people found a way to make some rather good mods, such as desert combat? I'm not a modder, so i don't really know how this stuff works.
Well, I'd imagine that fairly simple CS:S mods might start to appear but we won't be seeing modifications for any other Source game until they're actually released (oddly enough).
It's true that an SDK isn't really needed, but modding without one is the equivalent of trying to eat a large, hot meal with a pair of icicles instead of cutlery. You often end up using your fingers and making an amateurish-looking mess