Relatively small install base means a large potential amount of customers out there... a recent price cut on the system and sales making it even lower than that, making it look like more of an impulse 'don't want to miss this chance' pickup. An amazing weekend would be par for Sony, anything less than that would be a total failure.
They're spinning up the press for it too... saying things like 'it has PS2-like momentum' or something similar, and now they'll release something mid-late next week touting the x % of sales over their normal weekly amounts as if it wasn't Black Friday and talk again about 'turning the corner' and 'holding their breath'.
This is like seeing rainclouds and saying it's going to rain. If it doesn't, there'll be massive disappointment. If it rains, it was expected.