You're correct in looking for a concealer with SPF protection for your melasma. Melasma, a condition where one experiences dark patches on their face is one that can be a bear, especially for those who love the sun. Remember that you need broad band protection, and that you should reapply your sunscreen on a very regular basis when you're out in the sun.
As for concealers, there are a few good options for you. First, MAC Cosmetics has a wide array of colors in their Studio Finish SPF 35 Concealer. (buy direct). Plus, the high SPF coverage is nice for those with Melasma.
Next, Ko Gen Doh Foundation Color Corrector (buy direct) is a creamy concealer that comes in two shades and has SPF 16 protection against UVA and UVB rays. You can add this to your foundation or apply it right to your skin to lighten discolorations.
Finally, if you are a mineral makeup girl, you'll be happy to know that mineral makeup has natural SPF coverage. Bare Escentuals Multi-Tasking Concealer (buy direct) comes in five shades and has SPF 20.