On start, akonadi complains about missing executables: Executable "akonadi_nepomuk_feeder" for agent "akonadi_nepomuk_feeder" could not be found! Executable "akonadi_folderarchive_agent" for agent "akonadi_folderarchive_agent" could not be found! KDE version is 4.12.97 from kde-overlay.
On start of what? Which version of akonadi? Does this still happen with 4.13 final? What is your emerge --info please?
Yes it does happen with 4.13
You might need to enable the nepomuk USE flag. Please reopen if someone can provide more information, such as answers to questions from comment #1.