See http://docs.mongodb.org/manual/tutorial/build-indexes-in-the-background/

States:
"By default, MongoDB builds indexes in the foreground and prevent all read and write operations to the database while the index builds. Also, no operation that requires a read or write lock on all databases (e.g. listDatabases) can occur during a foreground index build."

Should be:
"By default, MongoDB builds indexes in the foreground and prevents all read and write operations to the database while the index builds. Also, no operation that requires a read or write lock on all databases (e.g. listDatabases) can occur during a foreground index build."