It is not so common, but it happens that web hosts are offering unlimited disk space. The reason why you don't see it so often is that it is very clear to everyone that a hard disk is limited. You can't fill an 80 gb hard disk with more than 80 gb, and the unlimited hard disk is not invented yet.
Unlimited disk space may be offered by hosting startups who does not understand what they are promising, or they plan on suspending any account that grows too large "Your web site has grown out of our hosting solutions". Unlimited disk space may also be offered in unlimited plans*.