Well, as far as I know, the sensor bar isn't a sensor bar at all. It's just a bar with a few IR-leds in it. The camera on the wiimote sees those leds and uses it to calculate what part of the screen its pointing at.
So the technique is very different from a normal remote, where there's a simple sensor on your TV that responds to simple pulses.
But with normal leds, if the sun or another bright lightsource is shining on them, it's often hard to see if they're on or off. I presume it's the same with IR-leds and IR-lightsources.
So there's not much they can do to remedy this short of totally changing the way the wiimote works.
But that's no problem for me, as I don't very often see that blue room with the fireball anyway.