This question is related to my problem in understanding rebase, branch and merge, and to the problem
How can you commit to your github account as you
VonC's answer is best, but the part that worked for me was super simple and is kind of buried among a lot of other possible answers. If you are like me, you ran into this issue while running a "getting started with rails" tutorial and you had NOT setup your public/private SSH keys.
If so, try this:
$>cd ~/.ssh
$>ls
If the output of ls is known_hosts and nothing else, visit: http://help.github.com/mac-key-setup/ and start following the instructions from the "Generating a key" section and down.
After running those instructions, my "git push origin master" command worked.