Bookmarking/place holder in a thread

On another website, admittedly a pay site, there is a feature where you can be taken back to the exact spot you left off in a thread. It keeps track of the threads you read and when there are new posts in a thread you read, an envelope next to the thread lights up green. You click that green envelope and it brings you directly to where you stopped reading. Now like I said thats on a pay site and they have much fewer users than TD. Its a really nice feature though and maybe something TD can adopt in the future.