This subject comes up every now and then on this forum. Some people report no adverse effects, others claim that weight lifting did hurt their pianistic abilities (for various reasons). To be fair to both sides, I would recommend to go about it in a reasonable way, don't overdo it, start very gently and build up very gradually, let your muscles rest at least a day between workouts, and be observant. Make note of any changes. Muscle can build up quite quickly. The movements required for playing piano may therefore have to be adapted slighly. Be extra careful with exercises that put a lot of stress on your fingers, wrists, elbows and shoulders, well, any joint for that matter. Again, build up very slowly and gradually. Also, don't forget cardio-vascular workouts (if you are not doing them already).