I opened up google maps to stare at it for a while, and one thing that jumped out at me is that I think they might be using different algorithms for different countries. Using what comes up as the default zoom for me (four notches from the bottom, enough to make the USA about 600 pixels wide) I see that the US has cities marked relatively sparsely, whereas they're much denser in Mexico.
Scrolling down to Australia it looks like the algorithm is far from perfect; it shows all the state and territory capitals except Adelaide, which is an odd omission.
More amusing, for the last couple of weeks Google Maps has placed Randolph Vt right in the middle of Lake Champlain.
Actual, maybe the cities are being hidden due to label placement? That is, there's not enough room to display all the cities with their labels, so rather than have them overlap, the algorithm chooses to hide the city if there's no room to place it.
I bet if you zoom in on a missing city it'll show up.
