The San Francisco Chronicle provides this concise explanation of the issue in Ashcroft vs. Raich, the medical marijuana case that will be argued today at the Supreme Court:

The question before the court is whether individual patients -- and, possibly, some of their suppliers -- are immune from federal enforcement.

The argument goes like this: The Constitution authorizes Congress to regulate interstate commerce. But no interstate commerce is involved when patients, acting legally under state law, use marijuana that was grown within the state and supplied without charge.

The counterargument, by the government and its allies, is that all illicit drug use affects interstate commerce. Even freely supplied marijuana boosts the demand for the drug, reduces the overall supply and may affect the price, the government says; in addition, pot looks the same whether it's grown locally or shipped between states.

For an exhaustive case primer, you need look no further than Drug War Rant.

We still haven't run across a link where you can listen live to the oral arguments. If you find one, please post it in the comments.