"Once you add the removed hosts back into the set, they will will detect that they have been added and synchronize to the current state of the set. Be aware that if the original master was one of the removed nodes, these members may need to rollback."

I was informed that a full resync from the new primary is necessary to prevent data corruption. The secondaries may not correctly enter into a rollback state.

I think that the "break the mirror" technique (described for versions < 2.0) still applies in V2.0 and beyond.

http://www.mongodb.org/display/DOCS/Reconfiguring+a+replica+set+when+members+are+down