Hey Bill, I asked the same question a few months back and didn't find a good resolution but this remains a question for many of us. Although this will not restore the felt to the whiteness of new fallen snow, here's what I've come up with lately:

1) Get a medium-flex CLEAN steel bristle hand brush.

2) Brush the felt straight up and down parallel to the shaft. You'll see the gray-yellowing start to come off.

3) Next, brush around the circumference. More graying is removed and the felt is becoming "fluffed up" a bit.

4) Finally, shave off the fluffed-up felt with a dry Bic razor (or several !). I've done this a few times and the beaters have improved from pure grunge to quite acceptable.

Michael