Bartek
This topic has been discussed here before, and you might experiment with a search to get other answers (and opinions).

And the other side of "If there isn't a contract, your client doesn't have any legal document to proove you work for him thus you can't be harmed." is If there isn't a contract, your client doesn't have any legal document to prove you work for him and so doesn't need to pay you."

In essence, though, yes, your client can sue you if you don't have a contract. An implied contract would exist and a lawsuit in this case would be based on emails, faxes, phone calls, receipts and invoices, depositions from each/all of you and associates, evidence of work you've done or not done.

It is incredibly time consuming and expensive to be caught up in a case like this. Even if your client decides s/he will never win such a case the opportunity to harass you by making you take time to locate appropriate documentation or explain why you don't have it can...well, it can sink your business.