IMHO there's a netiquette issue, apart from a legal issue. My suggestion is to let alwais, and as soon is opossible, the "owner" know that "his" work has been uploaded into Commons, thanking him and encouraging him to use wikipedia derived work in the terms of our free licences.

Many "owners" would appreciate it, some would claim; claims should be managed on common sense grounds. But it's far better a prompt claim, that a legal claim that comes after years. Isn't it?

Alex