As piwi needs a database backend to work correctly, either the mysql or postgres use flag needs to be used when it is emerged. Currently it is possible to emerge it without either useflag set which results in a package that, while sucessfully emerges, is not usable. Suggested solutions are to either automatically enable one if the user doesn't, or fail if both useflags are unset with a message requesting the user enable one and also rebuild app-admin/prelude-manager as it will need the given sql server support too.
Throwing it at the herd-name so everyone can follow.
Just commited net-analyzer/piwi-0.8.0.20031109-r2 that fixes this. Thanks for the report.