[greglindahl]

Federal laws frequently explicitly preempt state laws. When CAN-SPAM was passed, it invalidated California's stricter spam law.

California has stricter auto emissions because Federal law allows it. Other states are allowed to follow the California standard, because Federal law allows that.

[ams6110]

Mostly federal laws prempt state laws by threat of witholding some sort of funding. When the federal interstate speed limit was 55mph, states could still have higher limits but they would lose their federal transportation funding.

[Cidan]

Yep; these are very specifically crafted situations though, which is why I said most and not all.