But the light will come back after a few days. You still need to fix the problem. Sadly...
This is where you lost me:
What problem?
If you have an older truck with the switch, the light comes on when the odometer hits predetermined points at which time you reset it by moving the switch to the opposite position.
If you have an ODBII-equipped vehicle (which I think that you do not), the Check Engine light is extinguished by resetting the ECM with an ODBII reader or by disconnecting battery.
In either case, if a problem that triggers the Check Engine light (other than switch) is not fixed, then the light will come on again in a short period of time (minutes or seconds).
Referring to the original post at the top, you said that the light came on at exactly 100,000 miles. That indicates to me that your truck (pre-1996 I bet) has the switch under the dash that will, when placed in the opposite position, will turn the Check Engine light off. Further, if the light stays off, your engine is okay and you don't have anything to worry about if you perform regular maintenence and otherwise keep everything in good shape.
Hope that this helps!