I've heard that around southern Ontario, ATC boundaries cross the border
all the time. Detroit ATC controls airpsace above Windsor, Ont, AFAIK.

I remember talking to Toronto center for most of the leg over northern
Michigan, landing at Sault Ste Marie (still on the MI side). Its
probably opposite in other spots, like you say, maybe depends on radar
coverage due to geography, prevailing traffic, etc.

One funny story on that trip was that coming back the same route from
Ottawa, I was using Canadian enroute charts for a segment from Sault
Ste Marie to Rheinlander (WI), which, eventhough current, didn't
reflect a not-so-recent airway change. The American controller kept
telling me I was getting of course, and I was getting increasingly
confused. We finally sorted it out, the new airway had the same V-
number but bypassed a VOR where it previously made a dogleg. So I
guess that's a lesson for border flying, use the home country's charts
if you can, the system may seem transparent around the fence, but some
changes can take a while to propagate to both sides.