From Fedora Project Wiki

Are we still planning to have the Echo theme available for 10? I heard we would use the Mist theme, and updated the Gnome tour in the userguide to use the Mist theme. Probably should switch that to Echo if the Echo theme is going to make it. --Jjmcd

Official decision hasn't been made yet, but for 99% mist will be the default in F10. Echo will stay available as an alternative icon theme.

Thanks. I'll leave Mist in there unless you tell me different --Jjmcd