Don Bourdreaux argued a couple years ago that the primary reason that alcohol prohibition ended in the 1930s not because people thought it had failed but because the federal government needed more revenue. If he is right, there should be a good chance that use of marijuana will be legalized and taxed in the next few years because with the large increase in government spending coming down the pike, the government will need all the tax revenue it can find. In addition, if marijuana and perhaps other illegal drugs were legalized, there would be a lot fewer people in jail, cutting expenses of catching, convicting, and maintaining them.
Update May 13: I got this one right. See here.