A co-worker forwarded me this link to a post that describes the problems with Windows XP license keys not working after XP SP3 is slipstreamed into the installation media. The problem stems from the API calls made by the slipstream program when run from a Windows Vista machine.
He was using a volume license key in an answer file, but when the slipstreamed CD got to the point where it would read the file for the license key, the unattended installation kept asking for a valid key. Even after a valid license key was entered manually, the installer kept prompting for another key.
The problem was solved by performing the slipstream process from a Windows XP machine, rather from the Vista box. Apparently all types of license keys are affected, not just volume license keys.