First post of 2009, not too late to wish you all the best for this year!
Even if IPv6 is at our doors, the version 4 of the Internet Protocol will still be present for a (very) long time.
IP is an old protocol and lot of vulnerabilities were already discovered (and maybe others will be discovered in the future). Keeping an IP stack secure (or develop a new one for specific devices) is not easy. There are lot of information spread across several documents. Some vendors do not keep their code up-to-date.
Fernando Gont published an IETF draft about security assessment of the IPv4 protocol. This is a must read for all developers who are busy with IP stacks maintenance or if you are interested in the way how IPv4 works.