In case oolite-bin (see bug #106890) ever enters the portage tree, a 32 bit version of libart_lgpl will be needed to run the game on amd64.
OK, I just checked http://www.gentoo.org/proj/en/base/amd64/emul/emul-content.txt and it looks like libart_lgpl 32 bit libraries are included in emul-linux-x86-baselibs. I am now closing this bug as INVALID.