Now that Prop 19 has failed in California, a strange battlecry has returned to the Internet:
"Legalize marijuana and tax it!"
Two things come to mind:
First, don't worry, they would never forget to tax it.
Second, if they tax it enough the violent black market for marijuana will still exist. Imagine if they taxed it $500 an ounce. Clearly most people would continue buying it from drug dealers and nothing would change.
The stuff grows like a weed, remember, so anyone with a sunlamp in their closet can harvest it. The tax needs to be low enough so that the trouble of buying it illegally isn't worth the risk.