The Apple Keyboard II (second-generation short keyboard, shipped with the Classic/LC and most subsequent models that didn't come with extended keyboards) doesn't yellow on the spacebar.

My spacebars always wear (texture-wise) on the right side. My right thumb is somewhat arthritic and I adapted my typing style around it, using my right index finger to press the space bar. It's the only finger I ever use on the space bar so there is an obvious spot from it on most my keyboards. For some reason my Plus doesn't seem to wear much on the keyboard--but the new MacBook has.