I think this has been brought up before, but I thought I would post a link that clarifies the ATF and their stance. Before it was said that unless the person purchasing the firearm was clearly under the influence or used the medical marijuana card that as identification that they would be allowed to purchase the firearm given that everything else checks out okay. Now this is saying that anyone who is in possession of a MMC must say that they are a unlawful user of marijuana because in the eyes of the federal government anyone using marijuana is unlawful because the federal government still considers marijuana to have to medical value.
There are some lawsuits in the process, but I have heard from some reliable sources that ATF is planning on getting records of those in possession of firearms and a MMC and taking appropriate action, taking away their firearms.
I personally don't smoke, but I am a strong believer in our second admendment. This just looks like another way for the federal government to take away another one of our freedoms in the sake of "public safety". Here is a link for those interested. I would be careful if you have a MMC and go to purchase a firearm.