“Affinity Estimating is a technique many teams use to quickly and easily estimate (in Story Points) a large number of user stories. This is a great technique if you’re just starting a project and have a backlog that hasn’t been estimated yet.”
I remember this topic came up during the ScrumMaster course and I found it quite interesting, I’ve finally found an article related to it on Kane Mar’s blog, have a look at the entry here. I’ve yet to try it but will when I have a project with a huge amount of stories, it sounds like it can work quite well.