Also: at what point is memory usage worth worrying about? Scrolling for pages on end has to be tough on a browser.
A presentation that goes into some of these results is here: http://mcfunley.com/design-for-continuous-experimentation
On the cons side, as others have mentioned, if you navigate away from an infinitely scrolling page, then click the back button, you often don't land in the correct place.
Well, maybe; mobile browsers are often more constrained in terms of RAM. My Android phone only has 256MB, so infinite scrolling tends to cause the browser to bail out.