I've been looking into this very subject myself. I haven't looked about wellbutrin and phentermine.
Some doctors are prescribing "Phen-Pro" for weight loss, which is Phentermine with either Prozac or Zoloft. I have read some articles that say it is not safe, but other articles say it is being used for weight loss. Phentermine increases serotonin, while Prozac and Zoloft inhibit serotonin re-uptake. The combination can cause an overage of serotonin and lead to serotonin syndrome, which can be dangerous. See my other thread on Phen with Zoloft in the my experiences area.
I think wellbutrin is a different type of anti-depressant, not an SSRI, because I know that alot of people take wellbutrin along with zoloft. So, I don't know if wb is ok with phen or not.