SEULTRA,
I'm definitely not a legal beagle, but back when I lived in Virginia we had state safety inspections (every 6 months when I first moved there, later changed to once per year). After Federal emissions laws were enacted, part of that inspection included verifying the presence of the catalytic converters and fuel filler pipe reducer (for unleaded fuel) on cars. So even though these were a Federal requirement, the checks found their way into state inspections.
I would guess that the most likely places to start inspecting bikes for compliance would be California and those northeastern states which have enacted emissions regs similar to the California model. Any state, or portion thereof, which fails to meet EPA air quality standards can be compelled by the EPA to set up vehicle emissions testing. Northwest Indiana, where I live, is one of those "non-attainment" areas, and we have testing for cars at this time, but nothing so far on bikes. As for noise, I figure any state (Indiana) dumb enough to legalize the sale and use of fireworks (I'm not talking firecrackers and sparklers either) can't be too serious about noise pollution. I won't lose any sleep over that part of the equation.
Jerry