"Do you know what sort of temperature it takes to have Magnesium catch on fire? 883 degrees F.... Basically even if the battery inside the Surface caught fire or exploded the temperature wouldn't be high enough or last long enough to actually ignite Magnesium."
Lithium ion fires can be in excess of 500C, so it could theoretically hit the auto-ignition temperature of the case. Will it? Likely not, if anything the first thing to give will be the screen, which would likely drop the temperature to below ignition levels. Not to mention that if the battery exploded/caught fire the case damage is the least of your concerns