@Bob, not exactly. A huge majority of the AU searchers are indeed logged in (what my stats calls "secure search") and presumably getting personalized results. Of those AU search queries I can see, they're the same queries I do well on in the US.
The pages the AU/UK visitors are hitting do not contain anything that could be mistaken for a place name. I don't see any other way they could be mistaken as local, either - they are informational and written in American English.
It's like I'm suddenly doing well with logged-in AU/UK users and less well with US users in general despite no detectable changes in ranking. There are also no Adwords ads on the pages where I rank best.
Now, I am also down with Yahoo and Bing and other sources, and when I compare yesterday to July 9 of last year, I'm down about 65% on Google.com. So on the one hand, I think user behavior may be changing, but on the other it really looks like something's gone horribly wrong for me on Google.com... but it's not my rankings.
I'm completely at a loss.