I've had this exact same problem a few days ago. I eventually managed to solve it. I'm not quite sure how, but I'll tell you what I did anyway. Maybe it'll help you.
I started by downloading RVM. If you aren't using it yet, I highly recommend doing so. It basically creates a sandbox for a new separate installation of Ruby, RoR and RubyGems. In fact, you can have multiple installations simultaneously and instantly switch to one other. It works like a charm.
Why is this useful? Because you shouldn't mess with the default Ruby installation in OS X. The system depends on it. It's best to just leave the default Ruby and RoR installation alone and create a new one using RVM that you can use for your own development.
Once I created my separate Ruby installation, I just installed RoR, RubyGems and mysql, and it worked. For the exact steps I took, see my question: Installing Rails, MySQL, etc. everything goes wrong
Again: I don't know for certain this will solve your problem. But it certainly did the trick for me, and in any case using RVM is highly recommendable.