I'm sure it's technically possible, but would be difficult to administer. Imagine how many people sign up and then leave here, some the next day. You'd end up with hundreds of thousands of guests, which wouldn't really solve the problem because once a profile is deleted (wild speculation here) it has no need to be indexed, because guess what? It's gone, lost to the ether! Unless people were assigned a guest number when they singed up, I fail to see how this could be implemented at all. It would also look really rather untidy. Imagine it: in the forum when you see Guest 48 (okay, hyperbole), that would take up more space and characters than is allowed for an actual username.
