I have had plenty of radiator leaks (top hose) where coolant leak directly on manifold and have never seen it burst into flames (horrible smell yes). Not saying it didn't happen but with a 50/50 mix I don't seen how that is possible. Even funnier is that there is all kinds of warning about poisoning on Antifreeze containers I don't remember ever seeing a "flammable" warning. Hummm
Have a look at Section 5 of this MSDS
http://www.havoline.com/images/products/pdfs/anti_exlife.pdf
This material will burn although it is not easily ignited.
Not classified by OSHA as flammable or combustible
That's because there isn't one. At least, not on the container in the trunk of my car, right now.
I just went and looked.
There are boldly lettered warnings not to swallow nor to inhale the fumes (drinking it can be very fatal). The bottle gives the boiling point (265 degrees I think) and of course it warns you not to open the cap on an overheated radiator. There is not one single word anywhere on the container regarding flammability.
That tells me that the manufacturer considers the possibility so remote as to be next to impossible. If they thought there was any way on God's green earth you could set the stuff on fire, they would include a warning, just to protect themselves legally.
Again, this is the ordinary everyday coolant most people use; if there is something different in BMW's that is more flammable, then I'd like to know about it.