I think it should be raised in bugs.maemo.org for three reasons:

1. It is typical to report bugs to your provider, who is then responsible for liaising with upstream (admittedly, here this is a low-level developer-focused bug, so this may not apply so strongly)
2. This is more acute for Maemo - and especially in fremantle - where space is restricted and the button size in a dialogue box (where this text is most likely to appear) is fixed.
3. This should be fixed for the fremantle release, and it going upstream to some future Gtk+ version (if at all) is not going to help Maemo developers and users.

From point #3, a bug could be raised saying "fix bug #FOO from bugs.gnome.org in Maemo 5 Gtk+ before release", but there's still then a bug in bugs.maemo.org, just proxying upstream.