Chad,
A plug will tell you if you have a seal leak, detonating, or other details about your motor. When it comes to jetting, many will take the plug out and look at the insulator and make judgments based on the color to determine the next step they go with the jetting. This is a bad way to tune. Some will look at it and claim it's perfect...but what does that mean? It means nada.
Now, if you want to dial in your main jet, you can use a plug reading. But, you want to look way down inside the plug at the bottom of the insulator(you NEED a flashlight to do this, prefferably one with a magnifying sight). If you have ran WOT for a bit under a small load, then chop the throttle/hold kill switch, you will get a decent reading from the plug.
The bottom of the inuslator should have a very small black ring, say .5-1mm in height. This would indicate that your main jet is very close to being correct for that day, at that temp, at that elevation, at that particular relative humidity, using that particular pre-mix ratio.
Other than that, a plug will give you very little information on your tuning.
Check this out:
http://www.strappe.com/plugs.html