I have an account with the login 'david'. I recently changed my ssh key by updating ~/.ssh/authorized_keys on people and that worked fine
$ git clone ssh://firstname.lastname@example.org/git/udisks
remote: Counting objects: 4269, done.
remote: Compressing objects: 100% (1635/1635), done.
remote: Total 4269 (delta 3340), reused 3282 (delta 2620)
Receiving objects: 100% (4269/4269), 1.37 MiB | 888 KiB/s, done.
Resolving deltas: 100% (3340/3340), done.
However, when cloning from git.freedesktop.org it doesn't work
$ git clone ssh://email@example.com/git/udisks
Cloning into udisks...
Please copy the ssh key from /home/david/.ssh/authorized_keys from people.freedesktop.org to git.freedesktop.org. Thanks!
Please use the interface on http://www.freedesktop.org/wiki/AccountMaintenance for changing SSH keys.