Actually, random IP's is not only fairly common, but is the general rule for all ISP's dealing with a huge amount of users. As you all probably know, IP addresses are finite, that is, there's only a limited number available (which is true even with IPv6).
Most ISP's today also uses NAT (Network Address Translation) and PROXY-functions to accomodate multiple users behind a single IP-address. My point with this is that it's never a good idea to block an IP-address.
The worst case-senario is quite simple: if you and the guy you blocked uses the same ISP, maybe you've blocked yourself out.. Not very efficient... 🙂
// Olle