I actually don't use Adsense, but rather several other CPM brokers. The drastic drop since October 1 does seem to be across the board.
It's not just the shutdown. It's that if we default in a couple of weeks because of debt ceiling shenanigans, the markets could be horribly upset, Social Security checks won't go out, etc. If I were buying advertising, I'd be tempted to wait-and-see.