@Eric Mann - This is exactly what AutoMATTic is doing at wordpress.com. Infinite scrolling is being implemented site wide and there is no opt out except for cases where themes have footer widgets (apparently they have not found out how to retain those and have infinite scrolling). There will be a handful of the themes that will not get infinite scrolling because it makes no sense, but those will be rare. What's more, you can't even choose how many posts you want to show or per load. It is 7 and if you don't like that you are SOL.

One size does not fit all, and especially on image heavy sites where users might put multiple large sized images in each post, they typically will limit the number of posts per page to 2 or 3. Can't do that anymore. There are also those that want to have more control over how their stuff is presented and infinite scrolling simply doesn't work for them. AutoMATTic refers to them as "edge cases" and the law seems to be that you can then just ignore them.