For people with Motion + calibration issues, I honestly don't think it's a bug, but I know what causes it (in the demo). I never lose calibration while swinging the sword, running around, etc. It's caused by the Hero's Bow and the Beetle. Once you use either of those items, if you move your remote too soon (before the aiming reticle appears on the screen) on either item, unless you have your remote pointed at the dead center of your screen, it causes calibration issues. It's much like pressing down on the D-pad before you pick a zone to go to.
For example, say you want to use the Bow. You press B to use it and immediately move your remote somewhere that's not the middle of the screen (or where you were pointing when you pressed B). Once the aiming reticle shows up, no matter where your remote is pointed at that time, the reticle for the Bow always shows up in the middle of the screen (or wherever your remote was pointing when you pressed B).
Actually, I suppose this is still a bug, but the people saying that the calibration just randomly messes up is simply untrue. If it is, then it's time to get either a new Motion + or a Wii Remote +.