The "mv linux-${OKV} linux-${KV} || die..." statement on line 412 of kernel-2.eclass in the src-unpack() function fails when $OKV==$KV. The solution is to put an "if [ linux-${OKV} != linux-${KV} ]; then" line around it. This bug was discovered when trying to install the new development-sources-2.6.1.ebuild but will probably affect any vanilla sources package.
this was brought to my attention last night but unfortunately i havnt had timne to commit the fix yet. I shall do so shortl, thanks for logging this fault.
fixed in portage