Good timing, I was just re-reading some of my old posts re: melasma when I saw your question. Above is a link to the chloasma/melasma forum, it's not very active but my posts there have info and some links.
I had melasma, I got rid of it by reducing copper in my diet and supplementing with zinc, MSM and vitamin C. It's gone now, and I've been out in the sun this summer with NO sunscreen - this would have been unthinkable for me even up to last summer.
I came off the pill last Sept - I believe it was contributing to the melasma and preventing me from getting rid of it (it keeps copper levels high). I don't know about IUDs. I wouldn't use one, but that's just me. The Mirena one seems pretty dangerous - there's a forum here for people who have had side effects, some of them are pretty horrific. Copper IUD's will impact on the melasma.
Hope this helps!