Extended RAC is a Tier 6 DRP technology that enable zero data loss in case of major disaster at a local site, and near 0 downtime if well configured and well managed.
It is an expensive technology because of the costs associated with the required infrastructure, dark fiber over long distances and redundant components on all hardware layers.
A strict change control procedure must be in place, with an identical test environment where all changes to the infrastructure and new code versions are carefully tested before implementation on the production site.
Oracle 11g provides new features that greatly help on implementing extended clusters, and makes it the version that best fit the challenge.
The syncronization between sites is obtained using ASM redundancy, reads are made locally at each site using ASM preferred mirror read feature, and in case of a local storage failure ASM fast mirrot resync make recovery much easier.
10g R2 and 11g Dataguard FSFO (Fast Start Failover) technology provide a similar level of protection at less cost and at greater distances, as it relays on TCP for communications, so it is convenient to consider it as an option when evaluating extended RAC for DRP.
This document: Extended RAC On Pure Oracle Stack Summary contains basic implementation guidelines.
More information can be found on these excellent documents: