IS doesn't have a full heated screen like a Focus or Mondeo. In fact no Lexus does. My IS has the wiper heated part, but that won't demist or de-ice a screen. Fortunately, the IS engine warms up quick enough (ie within 5 mins) to blow warm enough air onto it to demist, but you're gonna have to manually defrost/de-ice it.
And contrary to popular belief, hot/warm water will de-ice your screen just fine without cracking it. I'm a windscreen fitter by trade, and in winter I pour hot tap water over my van screen every day to de-ice it, and haven't cracked one yet. When it snows, and my car is sat out in it for a week without being used, I fill up a couple washing up bowls of hot water to pour over the car to get rid of the snow and ice from it. The difference is, that how water from the tap will be fine, but boiling water from a kettle is too much, and will crack screens (especially ones with defects like stone chips). You need warm/hot tap water, and you need to pour it from the very top of the screen so it runs down (because it cools as it does). And also, the hotter the water (ie, boiling) the faster it will refreeze (the refrigeration principal - the heat of the water de-ices the screen, but the heat is whipped away by the cold outside temp, allowing the screen to rapidly cool again, and refreeze. It's this very fast cold > hot > cold that causes glass to crack, although this is usually only happens if you have a stone chip, which is a stress point)