Gentoo was one of the first Linux distributions to support AMD64, so of course the fashion in which it is implemented in Gentoo is incompatible with the de facto standard. The little gentoo penguins mistakenly designed a system that would be 64-bit with a little support for 32-bit 80x86 programs, but other distributions decided on a "multilib" approach where you put some "lib64" directories alongside the usual 80x86 "lib" directories. The multilib approach is prevailing, I think, because it is better, and in any case Gentoo for AMD64 is going to multilib.
But it is doing so by degrees, and now I have a chance to test the next phase of the transition, with a rescue CD handy.
This could go very, very wrong. <tt> :) </tt> Actually it doesn't look to me as if there is anything here I couldn't reverse, if I had trouble early, or work around, if I had trouble afterwards, if I had to.