Healthboards is busy health forum, not sure if this apply for its Skin problems category. Medhelp.org provides dermatologists' answeres. If it's a health problem, I can maybe provide some further information, would need to know what the problem is though.
These forums have loads of posts about skincare: style.com models.com ultimate-cosmetics.com Lots of beauty forums have skincare sections, search beauty forum on Google. HTH
There are Yahoo and Google groups out there, and everyone can search for "skin care forums". There are several reasons why someone want to join such a forum, so (by my experience), you can easilly find what you want through search engine.