I've never had an ipod (too mean to buy one) but maybe it has a signal strength bar graph or similar indicator? Could you buy another AP and site that one downstairs? Antenna Theory is a nightmare. So many things to consider: reflections from mountains, houses, walls or, worse, things which are unpredictable and can't easily be modelled. Organic objects like trees which cause seasonal changes to propagation, depending on leaf coverage, even rivers and waterways which rise and fall, affecting radiowaves differently. It's not 802.11 but when a cellphone operator (GSM/GPRS/UMTS etc.) installs a new mast to improve coverage, their team will have already spent many many hours at their desks, studying the topology of the landscape to model the likely propagation of their signal (come rain or shine). And they still get it wrong from time to time!
cheers, a