For what it's worth: The president can instruct the attorney general to non prioritize many aspects of the law. For example, if he were to instruct him not to prioritize murder, people would still be prosecuted in their states for murder, due to it being a state based capital offense when the situation is equivilant.

On the other hand, it would be a pain in the ass to prosecute murder on a federal level, which could lead psycho murderers to cross state lines etc... This would be termed as, beaurcratic red tape. In doing something like this, republicans would have all the ammunition to get an impeachment rolling, and it would all but guarantee a republican winner in 2012. Therefore, it will never happen.

But, in comparrison, lets say he instructs the AG not to priortize cannabis "use". This would actually do nothing to reduce cannabis arrests per say, mostly because possession is prosecuted by the state, and not the federal government. Instead, all it would do is allow states to follow suit, and would most likely put it on all ballots where applicable.

So, as previously mentioned, cannabis prosecution really begins and ends with the state. Unless you are a big time dealer! A non cooperative AG in regards to marijuana laws only means big time dealers are at greater risk of prosecution.