I thought about this just today as I was working with cement products (mortar and a cement based white paint). I had a few buckets of water with both lumps of cement and diluted cement.
Pouring it in the drain is a really bad idea, specially if the cement has not yet hardened, as it might harden in the sewage system.
As far as I know there should be nothing toxic in regular cement, except for its corrosive properties when not diluted.
I chose to pour the bucket out on a piece of grass, and when the summer comes and have dried the whole thing up, I will pick up the rubble with my hands.
Afterwards I realized that an old piece of fabric could have been used as a filter. Just put a towel on the grass. Pour the bucket on the towel. Either throw it in the trash now wet, or wait until it has dried up.
At construction sites I have seen masons just pour the jars on the ground, which leaves cakes of hardened cement here and there.