Here's one thread, at least. Try a search on white particles and white dust, as well, and you may get more, but it seems it's an internal piece causing it.
http://us.lexusownersclub.com/forums/index...p;hl=white+dust
This was a fairly rare issue when I had the problem in Sept 2007 supposedly on 3 other identical problems in the country. They initially replaced the filters but the problem returned. Then they replaced the evaporators twice and problem solved. My cars interior was gutted twice. Lexus regional sent techs to look into the problem so by now they are aware of it. Tell them to Call Lexus in East Hanover NJ and speak to their service and they can tell them the resolution if they can't get info from their regional techs.
Thanks for all the info. Your posts caused me to go back and check my service records. My service tech suspected the heater core, but now after reading the service report, I realize they actually replaced the evaporator. That was in December 2007. I should also mention that I took delivery of this 2007 vehicle in May 2006, soon after the RX350 was first introduced. So it was my second winter when the problem occured. And now I am just starting my second winter with the replaced evaporator and seeing the white dust again. Seems like a pattern to me.