About detached heads

This should be simple! I have a series of commits:

           1---2---3---4---5

I want to go back to 3 but not branch, so I want

           1---2---3---4---5---3

?

         git checkout 3...

gets me the commit on a detached head, but I don't know how to put this back
as the HEAD.

I'm on git 1.5.0.5

Cheers,
Geoff Russell