Had noticed a precipitous drop in my spam logs as well. It's just my personal domain but connections halved on Dec 27 and dropped down to 2 or 3 a day from the 28th to the 3rd. Normally I see around 200 spam connections per day. It has slightly increased from the 4th but even so I only got 17 connections yesterday (but most of these are from a single IP).
I find these numbers odd. If it was due to work computers being switched off I would have noticed a drop from the 24th. And if it was people not using home PCs because they had too many other fun things to do why do my stats for the 25th show it as an average day?
To me it feels more like someone throwing a big giant switch than anything else.