(Karoly asked me to report this.)
On the Debian install instructions page:
http://docs.mongodb.org/manual/tutorial/install-mongodb-on-debian/
There are nice clear instructions to setup MongoDB's 3rd party repo. Unfortunately, they're halfway down the page. When I first looked at the page I saw the first section and figured "oh, it's supposed to just be in the repo and I'm on an older version of Debian, so this won't work", then started looking elsewhere. Karoly then pointed it out to me and I felt dumb.
Please don't make me feel dumb!
A clear note at the top of the page that Mongo provides its own repo to use would be sufficient.