This reminds me of the related article I posted here on 07 July 2009 "Alarming Discovery on Sunscreen. Anyway, you can read the full article on sunglasses vis-a-vis cancer by clicking the link below:
The gist of this study is: "Sunglasses make the brain think it's dark and this means you're not starting the natural process of tanning. You're more likely to burn and therefore at more risk of skin cancer".
I reckon that the proper approach to this is to avoid the sun when it is at its peak usually from 9-10 to 2-3PM. The problem is when running an ultra race even during the hottest portion of the day. Personally, I would wear the proper clothing and sunglasses but only after I have exposed myself to the sun for several hours. In other words, I would expose myself, say from 6AM to 10AM so that my body would have already produced the natural substances for protection (melatonin, melanocyte-stimulating hormone, etc.) before donning the protective clothing and/or sunglasses.