I've come across reports of 7 seperate servers where after installing ISA 2004 SP2, the DHCP server does not work as expected. Reports are that the DHCP receive/request rules are in place but not functioning. The current resolution is to create a new set of DHCP receive/request rules.
Follow this article to create the rules, if you are having this problem. Hopefully later, I'll be able to post more on why the system rules are broken.