i've actually managed to do that once, but it seem kinda wierd

... i don't think it supposed to go of the backboard, it just happend to bounce on the edge of the backboard on a regular block..., so i don't think it was one of those 'real' ones that is in that G3 commecial...,
although i've managed to go up for the block and kinda get hit by the player and take the steal in mid-air...