As much as I love tumblr's auto pagination, one of the big gripes I have with it is that after a while of scrolling it becomes absolutely deadly to mistakenly click on a link that makes the browser leave the page. Because all that content was loaded with Ajax, there's no way of getting back to where you were except to repeat the scrolling orgy again.
history.replaceState to the rescue!
I submitted this as a suggestion to tumblr:
It often happens to me that I scroll my dashboard for a really long time, then click a link and forget to hold CMD to open it in the background. The entire scroll state is lost then when I get back to the dashboard. I think it would be a good idea to use the pushstate api to update the locationbar in a way similar to what happens if I click reblog on something. After I've sent the reblog, it takes me back to the correct place in my dashboard. Using pushstate would make this work even with external links.
This would also solve another problem: Sometimes, the loading indicator at the bottom of the page is stuck. In a normal webpage I would hit stop and reload and get my content but there's no way to do that here because then I have to scroll aaaaaall the way down again. With my approximate location stored in the url bar, this wouldn't be a problem anymore.
I think these features would also help you reduce the load on your servers a little because people wouldn't need to scroll over content they've already read as much anymore. Browsers not supporting pushstate could just keep working in the current way.
This will also work in GreaseMonkey. I might make a Safari Extension from this later.
Maybe I could solve this by watching the DOM for changes (as opposed to using Tumblr's handy callback hook) to manipulate the history but that seems kinda cumbersome right now.