Why are PS3's sitting on shelves? Well, duh... [update 1]
First off, the holiday shortages, wrought by blue laser diode production issues as well as the large amount of eBay hunters trying to snag huge profits (and some did). There simply weren't many to go around from November 17th to December 17th. What does this portend? Well, most holiday shopping is done between those times and since the PS3 was difficult to come by, many shoppers settled for the other systems. Namely, the 360 (since the Wii was also hard to find, but not due to actual shortages).
Secondly, after the holidays (we mean Christmas), PS3 production was ramped up. This would normally be good, but since the holidays just ended, people are recouping their funds, paying off that debt from holiday shopping, you know? Even if they see a PS3, they've no reason to slap down $600 or $500 for one -- there's no holiday to buy it for. It's also an irrational personal purchase at the time. January and February are historically slow sales months -- there's a reason hardly any games come out during this time.
Third, and most regrettable, is the severe lack of games for now. Since launch, there's been practically nothing on the PS3 front, hell, even the PS2 front is lacking in titles (but there are still a few goodies on the horizon!). Sony's probably going to pull out all the stops for the European launch. Tons of new titles all at once.
Hopefully this incites a little debate over why PS3's are sitting on some store shelves, but in all honestly, I truly believe these are the reasons why. It isn't because "Sony sucks" or "the PS3 is expensive" (partly true, but this reason is directly correlated to the time of year -- post-holiday), it's the reasons above. The next holiday season, when all systems are in ample supply, we'll see how that goes. Maybe it'll be different. Maybe not.
[update: Apparently the picture came from Kotaku, but we didn't get it from there. Whatever, credit is credit.]