Ok, just rang the DOT, and as per Motox's post they are legal. as long as they still comply with EPA standards etc and are not louder than 90db. They gave me the number of a guy who was supposed to be able to test my car for noise. He indicated that for my car the test they would do is a drive by.
The car needs to be at specific revs, in a specific gear and the valve has to blow at the exact time that it passes the meter... he didnt seem too keen test my car and commented that the cops were going overboard with defecting cars, and that the cost would be prohibitive ($500) (i think the DOT had given out his home number a few too many times.to spite that he was very helpful tho.)
He is faxing me the diagram and instructions for testing on monday, but said i would probably have to have it done by someone 'registered' if it were to have any amount of legal standing.
So as far as i can tell, they are legal, as long as EPA and noise requirments are met.
How you can prove it is another matter.
Thanks for your input everyone.
Cheers
Hayden