If you look at the transparency reports, only the report for the first half of 2013 includes that canary (specifically mentioning Section 215).

It was removed for the latter half of 2013, and this just-released first half of 2014 report.

Take that as you will.

I highly doubt the canary would be a valid defence.

By omitting the line you are, for most intents and purposes, saying that you received a request. Now, saying you received a request and saying specifically what request you received are different, but I imagine the gag order doesn't make that differentiation.

Gag order is not that you can't say a specific thing, it's that you can't communicate a specific piece of information. The canary is communicating something.

A court order can ask you to lie, so the "I can't lie to my customers" defence probably won't work.