When do you report an adult child as missing?

It has been six weeks since I have seen my son logged into 'MySpace'. Now, normally, most people that wouldnt be a big deal but he was on that darn page every day, several times a day. So this could mean a few things and of course my mind goes to the worst of them.

He could just not have access to a computer, but in the past he would at least find access every couple of days.

He could be incarcerated somewhere again.

Or the worst of the worst, something could have happended to him physically.

A few weeks ago, this overwhelming feeling that something is wrong, that he is either injured, sick or in some kind of danger just came over me...I guess mother's intuition. Since this time I have been toying with the idea of going to the police to enter a missing person's report. I just keep thinking that if something has happended to him and he has no identification on him how will anyone know who he is or that someone is looking for him.

I am not sure that the police would even take me seriously or that they wouldnt laugh me right out of their office, especially given his past history.

So when do you report an adult child as missing?