It's not an FAA regulation; most FAs have used the phrase "United security policy" for several months now to cordon off the FC lav.

I have been known to walk from row 3 or 4 of a 737 to the FC lav. I haven't been Tasered yet. Of course, I'm unbelievably attractive, so maybe that helps.

It seems to me that the announcements about sectioning the lavs serve two purposes:

1) They have a very, very minor security enhancement effect, as it supposedly prevents all 14 of the hijackers from congregating around the lav to plot how they're going to force a post-9/11 secure and impregnable cockpit door open. Note, I did mention this was a very, very minor security enhancement effect.

2) More telling IMO, it provides one last sense of entitlement to the FC pax. "You can take away my meals, my linens, my service, even any sense of seat pitch advantage...but that FC lav is MINE and I shall HAVE it!"