Lets Face it. Todays Databases are getting bigger and bigger and backups not only are painfully long depending on the methodology you are using but also can cause performance issues. While Oracle has allowed for various methodologies including Backing up from the Physical Standby and now IN 11.2 the introduction of Change tracking file compatibility for incremental backups with Active Dataguard. Companies that like Hardware solution also have a choice of using products like EMC Clones or IBM Flashcopy . TO use either of these the Frame must be an EMC or IBM Frame. These copies can be mounted onto a seperate host and Full backups can be offloaded to tape as well as a Local DR copy split to account for various errors.
Metalink Doc id #302615.1 provides and excellent overview of How to Use Rman with Split Mirror Technology.
This Document by EMC and Oracle also provides and excellent overview. While Old The concepts still hold true in Oracle 10 and 11.
I have had the oppurtunity to test the solution as well as perform extensive recovery testing to ensure the solution works