I have a lot of little polygons (block of houses, sezioni censuarie in Italy) that contain resident in a city. I have united the shape of the polygonsin Arcgis10, with Geoprocessing>UNION, to a grid of 1 kmq, so in every cells there are many blocks or sezioni censuarie, completes but not complits too. Every block have a population: how I can attribute the right pop, of the divided block to the grid? So I can redistribute the pop in the grid and show this with ArScene
If I have understood your question correctly, you want to sum the population of the individual house polygons for each 1km grid cell and then extrude the grid cells in ArcScene.
However, I expect that some of your house blocks may be on the boundary of two or even four grid cells. So, you will want to think about how you handle this situation. It may be that you first intersect the houses with the grids and calculate the population for the new (divided) polygons as a proportion of the original population based on the new area/original area. If so, obviously do this before the spatial join.