This is one of the most useful things I've found in a while. I don't know how I didn't find it earlier. This enables you to easily type things like éen, Khazad-dûm, Mikael Åkerfeldt, hwæt, Tromsø, etc...without having the memorize a number for every special character (à la 'Doze) or looking it up in a character map program.
For some reason, I can't get Vietnamese characters to enter, but still incredibly useful.
Edit: It would appear that Gtk uses its own built-in compose sequences with the Multi_key. You have to set the input method module to XIM to change the default behavior, which can be done with export GTK_IM_MODULE=xim
A little old, but see also: http://www.linux.com/article.pl?sid=06/01/26/210214