Some idiots ruin all the fun...
Mississippi was one of the dozen remaining hold outs until some idiots in a blown car decided to take some police on a high speed chase that ended in the death of Trooper Steve Hood. As such, NOS is now illegal in Mississippi street cars, and is the 39th state to pass such a law.
As far as the specifics go, no one's sure exactly how the law is written. We know (for instance) that in California you can have a nitrous oxide system in your car and the bottle can even be hooked up, however said bottle must be 100% empty. In other states you're not allowed to have the bottle screwed in. Still, in others, a NOS system is illegal period.
A little advice for the Mississippi residents from the folks at Bangshift.com: "Seeing as though this is a law fresh in the minds of law enforcement and represents an emotional event for the Mississippi State Troopers, we'd stow our bottle well away from the mount when headed to the race track."