From Sheldon:

"Threadless forks are generally supplied with very long steerers, intended to be cut to appropriate length for various different bike frame sizes. If you are buying a new fork, or a new frame, you can leave the steerer full length even if you ride a smaller frame. This will let you get the stem nice and high without needing any extra accessories. On some of my own bikes, I've gone this route, and have used a separate clamp to secure the headset, rather than using the stem for that purpose. A 1 1/8 steerer is 28.6 mm. Pinch-bolt type collars are readily available in this diameter, some of them intended for seat tube use.
Using the separate collar allows you to mount the stem anywhere on the extended steerer, without any need for spacer washers, and without any need to re-adjust the headset every time you raise or lower the stem."


to read more:
http://webcache.googleusercontent.co...&ct=clnk&gl=us

Bearing in mind that a carbon steerer is a whole different ball game (in my opinion, anyway)