Is this license plate cover illegal?

I live in NH...bought these license plate covers cause I like the look. Just realized these are probably illegal to have on your car in most states...and yes, it is probably a definite reason to pull you over. Here they are...gimme' your 2 cents. http://www.vettenuts.net/images/products/large_463_SmokeShield.jpg show more I live in NH...bought these license plate covers cause I like the look. Just realized these are probably illegal to have on your car in most states...and yes, it is probably a definite reason to pull you over. Here they are...gimme' your 2 cents.

http://www.vettenuts.net/images/products/large_463_SmokeShield.jpg
Update: The license plate cover in the pic is a little exaggerated..the plate is completely visible if you were to follow behind me.
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