Of all the internet myths out there, I'm glad this is the only one we have to deal with on a constant basis. Yes, Lipos are flammable. They are not explosive and will not burn unless heated enough for the cells to burst. That can happen if the battery is shorted, damaged, or severely overcharged. The first ones to hit the market were fairly fragile and unpredictable, but pretty much anything now that's new enough to even use is very trustworthy. Charge them right, don't keep them if they are damaged in a crash, and don't let them get shorted out (good wiring, quality ESC's) and they are no more dangerous than the box of matches in your kitchen drawer or the can of gas in your shed.