I just got my Cisco 851w and was interested in finding an answer with ddns, since I need it.
Here's what I found out on the Cisco command lookup tools
interface ethernet1/0
ip ddns update hostname testhost
Related Commands
Command Description
debug dhcp
Displays debugging information about the DHCP client and monitors the status of DHCP packets.
debug ip ddns update
Enables debugging for DDNS updates.
debug ip dhcp server
Enables DHCP server debugging.
host (host-list)
Specifies a list of hosts that will receive DDNS updates of A and PTR RRs.
ip ddns update method
Specifies a method of DDNS updates of A and PTR RRs and the maximum interval between the updates.
ip dhcp client update dns
Enables DDNS updates of A RRs using the same hostname passed in the hostname and FQDN options by a client.
ip dhcp-client update dns
Enables DDNS updates of A RRs using the same hostname passed in the hostname and FQDN options by a client.
ip dhcp update dns
Enables DDNS updates of A and PTR RRs for most address pools.
ip host-list
Specifies a list of hosts that will receive DDNS updates of A and PTR RRs.
show ip ddns update
Displays information about the DDNS updates.
show ip ddns update method
Displays information about the DDNS update method.
show ip host-list
Displays the assigned hosts in a list.
update dns
Dynamically updates a DNS with A and PTR RRs for some address pools.