Unlimited hosting on a whole isn't really possible and has been discussed here many times. Now when you're talking about Unlimited Hosting*, that's different. Notice the Asterisk? That means there's some sort of limitation associated with it.
When you read through the TOS and AUP you'll usually find that there's provisions in there that really prevent the unlimited nature.
That said, there is UNMETERED, and we use that here ourselves. We don't track the bandwidth used by a client, but that doesn't mean that they can use as much as they want - there's always a limit. In that case, it's limited by the amount of throughput the port can put out. It's also limited by the CPU and Memory and how much it can allocate to sending the bandwidth. We then have limitations in our TOS that prevent excessive memory & CPU usage.
We do offer "unlimited email accounts" however. This implies that a person can set up as many as they want, and technically they can (until they run out of space). Each folder generates 4k-8k bytes. If you generate enough empty folders, you'll fill up your disk space and then you're limited by the amount of disk space you're allowed to use, or the number of inodes you can use on a given machine etc.