In addition to wearing through the braking surfaces, aluminum rims will begin to crack, at the spoke holes usually. Aluminum doesn't have infinity flex cycles like the stainless steel spokes do, and since the rim is always flexing, it will start to develop cracks. But just like brake surface wear, the key word is "eventually". By the time the rims wear out you'll be ready for new hubs anyway and you'll probably just replace the wheels.