HTTP 1.1 pipelining

I'm looking into request pipelining & had a question about the kind of requests allowed.
The RFC says only idempotent requests should be pipelined.
This FAQ (http://www.mozilla.org/projects/netlib/http/pipelining-faq.html) from Mozilla says PUT should not be used because it isn't idempotent. Except that it is. PUT is idempotent (repeatable with deterministic results).

Which is it?