I have a set of Stan's alloy rims that are as light as some of the new carbon offerings that are coming out. They do have weight limits, as well as pressure limits. That said, I love them, and can't think of a reason, really, to switch to carbon (and I race, a lot). My entire bike weighs 22.5 lbs with these wheels and SPD pedals (it's a size L).

You might want to look at some other options that may be less expensive than carbon. As others have said, depending on your terrain and riding style, the carbon rims could be more prone to damage. I dented an aluminum rim last year on a relatively innocuous gap I have ridden hundreds of times.

SheFly