as long as the case is over so the CPU is facing upwards the hot air will still rise and be extracted by the rear 120mm.
However (looking at one of my P180's) if you do this, the door will open upwards and not downwards, although it will open right back on itself by the way its hinged, this is still a bit of a bummer.
edit: why you wanna lie it down anyway??
just get a desktop case like a lascala or alike...