There has been plenty of that on my floor. Generally a high concentration of weld spatter dropping on it will make the whole area look "baked"....kinda like a dirty spot that won't come off. With as often as I mop the rest of my floor, you can't even tell.
It doesn't burn a hole or anything, unless you drip a big glob. I have some sheetmetal I put down if I'm going to be doing anything seriously messy, like plasma cutting brackets off a frame or something.
Edit: the baked color only happens when welding close to the floor--underneath a vehicle upside down, or under an axle. Sparks from up higher, like on a workbench don't seem to affect it at all.