Index table locking

I have not kept up with PostgreSQL changes and have just been using it. A
co-worker recently told me that you need to word "CONCURRENTLY" in "CREATE
INDEX" to avoid table locking. I called BS on this because to my knowledge
PostgreSQL does not lock tables. I referenced this page in the
documentation:

http://www.postgresql.org/docs/9.3/static/locking-indexes.html

However, I do see this sentece in the indexing page that was not in the
docs prior to 8.0:

"Creating an index can interfere with regular operation of a database.
Normally PostgreSQL locks the table to be indexed against writes and
performs the entire index build with a single scan of the table."

Is this true? When/why the change?