What happens if:
-- you dump a private key from bitcoin client 'A'
-- shutdown A
-- import it into bitcoin client 'B'
-- spend it from B
... wait for a confirmation or three...
-- restart A
Does A notice that the coin's been spent? I think there's a bug that it does not, and I think that bug needs to be fixed before we make it easy to export/import private keys. So, please bang on sipa's patch and see if anything else breaks!