Who is going to risk all the Bitcoins he have, for the sake of 1% per month interest? The only established player offering interest on Bitcoin holdings is Vircurex, although their rates are not constant. I am having an account there, but I never hold any coins as Vircurex is not a safe holding place. It was hacked multiple times in the past.
Only safe place to hold long time is your wallet with a backup on an offline computer.. but you never get rich that way (or at least not fast enough lol)