Genkernel 3.4.20, which is currently marked stable, fails during the build even when no changes have been made to the default configuration of the 3.2.12 kernel.
I will post the exact message once I do a fresh install, so I can reproduce the error message(s).
It seemed to be related to NFS parts of the kernel. According to some supporters on IRC #gentoo, I could have either disabled NFS or used 3.4.24. I chose the latter, which turned out to work fine on at least 3 recent installations (different systems, all amd64).
Genkernel should not be regarded as stable in the current configuration; perhaps a version bump is required.
I just (after filing the bug) saw that in the meantime genkernel 3.4.24-r1 was set to stable; if this is the case, then the current stable combination should work fine in this regard, and I guess the bug can be set to resolved.
Perhaps someone would like to make a note to take measures so that a bug _like_ this is harder to slip through to stable in the future.
*** This bug has been marked as a duplicate of bug 413247 ***
This is indeed the same issue.