Just an extra thought, the dress maybe a little "clipped" where the light hits it strongly. The 50d has a function called Highlight Tone Priority. Did you have this switched on as it greatly helps stop this happening. This is a link to digital pro talk, they explain really well what it can do (I hope I am allowed to do this)

http://digitalprotalk.blogspot.com/2007/10/highlight-tone-priority-image-salvati...

Hope you dont mind the comment, but I still think this is a superb shot...Jerry