It has already happened on reddit last week. Someone came to /r/favors and offered a free premium account to a file hosting website he had just built. The deal was: you open an account on the website, you PM'me your username and I'll make it premium. Of course he got a load of users giving him data and then disappeared, along with the website.

Any other "HN Rate my webapp" thread could do the same.

There are ways of hashing nice password with custom-modified algorithms and bookmarklets. Use them guys.