The same thing just happened to me! We had a big winter storm in DC last week and my one week old 2010 X5 35d split its driver side mirror down the middle when I turned on the defroster. It now has what look like burn marks along the crack in the mirror.

This has to be a defect! The dealer (Passport BMW in MD) told me to swing by and they will check it out. I hope they cover this, as I will be pissed if they say it was my fault.