Recently I had to investigate whether Cisco Router and Security Device Manager (SDM) uses a default username/password, so I was forced to figure out how exactly the SDM works. The minimum functionality needed on an SDM-enabled router is preconfigured Ethernet interface, HTTP server and (optionally) DHCP pool. Just in case you need to reset a router configuration into a state where you can reinitialize it with SDM, here's the minimum configuration to make it work:

! Use whatever your first Ethernet interface is here
!
interface FastEthernet0/0
 ip address 10.10.10.1 255.255.255.248
 duplex auto

ip http server
no ip http secure-server

ip dhcp pool SDM
   network 10.10.10.0 255.255.255.248

Note: routers shipped from Cisco would usually include username cisco password cisco configuration command.