I tried using a belt to set a higher squat max. It didnt help at all. it actually hindered my lift. but then again, i had a relitively soft, thin belt. and it may have been to tight.
I dont really see much purpose in using a belt from a practical persepctive. because your leaving a weak link. Which means the strength gains wont transfer over as well into real-world strength. Like needsize said, his back got stronger after he stoped using his. However, as a bodybuilder maybe you could use it to your advantage. If your weak link is your trunk muscles. then your probably not able to challenge your legs maximally in a set of squats. As your back is not capable of supporting the weights needed to maximally challenge your legs. So if using a belt helps you do this then you could acheive a better workout for your legs, by eliminating the weak point with a crutch.
But really the best way to bring your lower back strength up is to throw in some reverse hypers, or partial deads, in replace of something less important or effective.