When configuring a route-map with the "next-hop dynamic dhcp" option, the dhcp gateway remained UNKNOWN until we rebooted the router. This makes sense since there is no mechanism to generate a dhcprequest for a gateway until the router is rebooted. Tested on Cisco 891, 881, 871, and 1841s, 12.x and 15.x IOSs. Works after initial reboot and all subsequent reboots.
We had search the Internet, Cisco documentation, and opened a TAC case and found no solution. A simple warm reboot will solve the issue.