I'm no expert, but maybe this will help...

Here's a link to some of my autocross photos. They're not bikes, but the technique for getting the photo should be the same. 99% of these shots were taken with a Sony P-51, 2.1MP, snappy camera, certainly nothing fancy. ISO was set at 400 to get the fastest shutter speed possible, no other controls are available on that camera. The cars are typically moving 20MPH or so, in most of those photos.
Autocross photos
You'll see that nearly all the shots are clear and usably sharp. The key for me was to pan the camera while taking the shot. This means tracking the subject with the camera and pressing the shutter while panning. I continue to pan the camera until I'm sure the shutter is finished. Also, I have to deal with a little shutter lag, so I have to press the shutter a tad early. A little practice helps here....
Another thought is the angle of the camera vs. the moving subject. I have better luck shooting perpendicular to the passing car, simply because the distance from the camera to the car remains relatively constant during the pan, making it a bit easier on the autofocus system.
Head-on shots of a fast moving subject is likely to cause some issues with autofocusing, depending on the speed of the subject and the lag between focusing and actuating the shutter. A continuous focusing system may help in that situation. I can't wait to try my new FZ-20 next autocross season for this reason.
Sorry to ramble, I hope there's something in there that helps!
Good luck,
- Scott