I have been taking Adderall for a year, 20 mg twice a day. I'm also taking Wellbutrin 150 mg 3 times a day, and Levoxyl 100 mcg once a day. I'm now a graduate student, and have been a certified pharmacy technician since 1995. I drink occasionally and have not noticed any problem. I'm a 47-year old overweight female and my blood pressure has been OK (these meds and alcohol could increase BP. What I do is read all the package inserts for all medications I'm taking, and then call or speak to a pharmacist. They're the experts in drug interactions, and usually they'll end their comments with "ask your doctor". I don't always follow all advice, but at least I'm informed.
Taking Adderall will definitely decrease your tolerance for alcohol. Specifically why, I'm not entirely sure, but I would expect it to have something to do with it being a stimulant and therefore allowing your body to move the alcohol into your bloodstream. This, of course, will lead to becoming intoxicated much sooner than under "normal" (unmedicated) circumstances. For instance, unmedicated, I can consume ~6 beers before feeling the effects; when on Adderall, I feel these effects after 2 or 3. If this is a attributable to Adderall being a stimulant, I would suspect non-stimulant alternatives, such as Straterra, to not exhibit this effect.
There may be other consequences to drinking while on Adderall that I'm not aware of. Perhaps others will fill us in with their knowledge.