I don't know all that much about Wellbutrin. I know it's not an SSRI. From some quick reading, alcohol intake will increase the speed at which wellbutrin is metabolized, which (likely) would amplify the effect of the wellbutrin. Wellbutrin is a dopamine and norepinephrine reuptake inhibitor, and I think increasing the speed at which it is metabolized would probably cause unpredictable behavior. It'd basically be mixing a depressant (alcohol), which normally would slow down the effects of dopamine and norepinephrine, with a drug whose purpose is to increase those very effects.
Maybe Smooth can show up to drop some knowledge. Some notes online seem to indicate that moderate amounts of alcohol are okay, and there are also some indications that altering your intake of alcohol (ie. drinking a lot more than usual, or even CUTTING DOWN on the amount you drink) can also cause unpredictable behavior (since changing intake would alter the metabolism your body is used to, I suppose).
If you're talking to someone that's prescribing you wellbutrin, I'd be honest to him about your alcohol intake. Again, I really don't know wellbutrin, but it seems that different people have vastly different experiences mixing alcohol with it. I do have to say that it's the first antidepressant I've read of where they don't flat out say "do not mix with alcohol."