The browser does download subresources, including images, scripts, and stylesheets. The new page visibility API is intended to help folks keep accurate web stats:
So yes, this will screw up web stats. A new JS API won't help with anything server side, and there's going to be a significant lag before every web app in the world is updated. Think about preloading anything that keeps track of read/unread status as well (e-mail, forum threads, etc).