Allocated IPv4 space
At the rate at which IPs are being consumed, all usable IPv4 based addresses will be consumed by 2012. It is very important that organizations start switching to IPv6.
Currently, about 22 usable class A blocks are labeled as unallocated on IANA's IPv4 address space page. There are other blocks marked reserved, but these should be considered unusable because some network admins have even blocked them in their firewalls and consider IPs on those blocks bogons.