Never had carb icing issues on my XT, but I have on other bikes (Ducati. Honda): various symptoms from cutting out on idle, dying completely as you describe, and even carbs sticking open - really scary!!
In my experience it tends to happen when it is close to freezing, with lots of moisture in the air (mist/fog), rather than on drier, colder days.
Sometimes it's obvious ie the carb is covered in ice. Other times, leaving the bike standing for a few minutes is enough to thaw it through. Is that what you're seeing?
As XT carbs are rubber-mounted, and thus isolated from engine heat, they could be susceptible, but my 1VJ (with 3AJ air box) doesn't seem to suffer. Maybe the air is being warmed enough before it hits the carbs?