>>>>If some firm asked you your opinion about me and you'd give it (either negative or positive), without my approval or request for being a reference, I'd have bad feelings about it.
>>>
>>>Negative comments are illegal in the USA and Canada as far as I know.
>>
>>They aren't illegal, it's just that most corporate human resources departments avoid them because of potential legal liability.
>
>There have been published court cases. No, I will not look up the references.

Here is the first link I found by googling. It sounds the same as libel in general, that it isn't illegal if it's true.

http://injury-law.freeadvice.com/libel_and_slander/negative_job_references.htm