No chance it passes anytime soon. What would need to happen before something like this would occur would be for all 50 states to pass legislation similar to what they have in California and a few other states. Congress is not going to legalize something that many individual states have still deemed illegal. Even if all the states did then the feds still may not, the DEA makes too much money for them to end this war on drugs anytime soon. Plus marijuana possession is a big moneymaker for local police. I don't know if the tax benefits from selling it and taxing it like alcohol would make up for it but it's a thought.
I also think that the alcohol and tobacco companies have too much to lose by weed being legal. That would end their monopoly on intoxicating our citizens obviously, and they have a good bit of pull within our federal government id imagine.
Plus check out Bush's response to this if you haven't seen it....
http://rawstory.com/news/2008/Bush_d...ress_0730.html