But it's not a particularly exciting one, we're afraid. Oh sure, it might play havoc with your homebrewing, but you're not going to see any differences in how the system works. In Japan, 4.1 fixed a bug in Wii Sports Resort, apparently, so we're betting it's the same case here.
So go on, update. Or don't. Up to you. We just wanted to temper your excitement when you saw that telltale blue glow.