I'm on depo and about 3 months into accutane. I have never heard anything at all about accutane making depo less effective! I certainly haven't noticed any changes since being on accutane - still no periods and no spotting or any period related symptoms at all really. They accept depo as your main form of contraception when taking accutane, but they will ask you to use a secondary form like condoms for the duration of your course anyway, as they always require 2 forms of contraception (as far as I know). Acne definitely is one of the possible side effects of depo, but if you have been using it for a while and it wasn't causing you problems before then I don't know but it would seem unlikely that would be the cause. For me my derm thought it was more likely that coming off yasmin had caused problems rather than going on depo, but honestly I think that depo is having some negative effect on my skin, even if it's not the main cause. I also have depo for regulating (stopping) my terrible periods, and I love it so much for that, so I'm staying on it as long as possible!
If you are concerned about interactions then ask your pharmacist when you pick up the pills, but honestly not one of the many drs, nurses, or pharmacists I've encountered have ever said a word about it and they all know I'm taking both!

Awesome thank you!  Thats interesting actually... I was on Yaz before as well, I wonder if maybe that contributed to my acne at all.  Oh well!  I'm just excited to finally get to try accutane eventually, after all these damn pregnancy tests lol.  Depo has totally been a miracle drug for me, so its good to know that others have been on both without any crazy interactions or periods from hell