You can detect APs across a WAN, but there are a few gotchas.
If it is a T1 or lower, your only option is to use H-REAP mode, but even then Cisco recommends no more than 1 or 2 APs per WAN link. This is due to the amount of bandwidth required for the LWAPP management.
If you need more APs than 1 or 2, it is a very good idea to use local controllers.
Also keep in mind that remote APs would not be able to authenticate users while the WAN is down if you use any EAP methods such as EAP-FAST or PEAP. Current users would work until they time out, but no new users can authenticate since the controller manages the authentication process.
-Eric
