Federal law's basically a total dick about state laws getting crazy. If anything happens within the federal system they're not into, national law enforcers flip out.
The U.S. government still considers marijuana a Schedule I drug (no accepted medical benefits, high potential for abuse). So you could get a misdemeanor just for possessing it in any amount, a felony for cultivating or selling it, and even a felony for selling paraphernalia related to weed.
But hey, the DEA will probably just be chill about this Colorado/Washington thing, right?
America spends billions of dollars every year funding the War on Drugs. And while there are probably many reasonable, level-headed people within its ranks, the U.S. Drug Enforcement Agency spends thousands of man-hours killing wild hemp. That's right: the DEA hates "weed" so much that they're now afraid of every weed.
So those pro-weed state laws won't end up mattering, right? Right? Well, sort of.