You can do this in Bash already (Ctrl x Ctrl e is bound to readline's edit-and-execute-command, which will open $EDITOR with the current command), but being able to do this everywhere is nice!
For those of us who use Zsh, the equivalent is “Alt-x edit-command-line”, which can easily be bound to “Ctrl x, Ctrl e” by adding “bindkey '^x^e' edit-commandline” in ~/.zshrc.