Make sure you use weight training correctly so you don't end up becoming muscle-bound and inflexible without much strength. These are some of the most helpful tips for weight training.
Strength training will help prevent injuries and increase bike strength and power, important in hill climbing, time trialing, and sprinting. Mountain bikers especially should read this article...
Muscling up is something to be done cautiously. You must be strong, while maintaining flexibility and keeping your weight down to be a good cyclist. Here are some great ideas on how to do just that...