We do a disaster recovery of the entire domain, including a domain controller and all our application servers. The DR simulation takes place on a separate network, so it can be done while the production network stays online.
We have spare hardware for this purpose, which is kept at a separate site (where our backup tapes are taken to each day).
This allows us to simulate a total disaster, plu if we expand our DR preparations to include off-site replication, we already have the hardware.