Will a 80GB iPod case fit a 30GB iPod?

Yes. The only difference between the 80GB and 30GB is the harddrive's digital size, it's physical size is the same. This is all assuming that it is the same generation. You see, there is an iPod classic 5th generation 80GB and an iPod classic 6th generation 80GB, although both are extremely close in size and I have had a 5th gen case on a 6th gen iPod. Hope this helps. ~Griffin