Seems like what I wrote about Qt4 QtRuby and Qt3 QtRuby was incorrect, or at least outdated, as the current version of Qt4 QtRuby does install a differently named shared object (qtruby4.so), which makes them compatible to install side by side.
Last time I tried it didn’t even use CMake as far as I remember.
Thanks Caleb for correcting me 🙂
Now the problem is that Qt4 QtRuby fails multilib-strict 😉 But that’s Gentoo-specific, probably.