It looks like the information on building background indexes on a secondary may be based on an incorrect commit comment for SERVER-2771:
http://www.mongodb.org/display/DOCS/Building+indexes+with+replica+sets#Buildingindexeswithreplicasets-Version2.1.0andlater

The comment from Kristina says:

Index builds will now happen in the background on secondaries if they happen in the background on the primary. This means that reads can continue while the index builds. However, replication will block until the index build is complete.

.. but the preceding commit actually appears to change the background index on the primary to a foreground index with the log message:

info: indexing in foreground on this replica; was a background index build on the primary