I've updated to 3.4-1 and among the rc.lua configurations that broke, I missed my titlebars the most.
I think I figured it out though:
if awful.rules.match( c, { class = "Gimp" } ) then
awful.titlebar.add(c, { modkey = modkey })
end
Unfortunately, writing "gimp" with a lowercase "g" didn't work. It had to be "g", even though:
WM_CLASS(STRING) = "gimp", "Gimp"