I'm still researching this now that it has come up again. It looks like there may be some policy regarding lavs but not about just entering another cabin. I'll have more shortly but in the meantime as to the issue of just being in a different cabin I found this from an old Cranky Flyer posting that got into this topic...

Steve, before “retiring” from the airline business, I worked directly with the Director of Safety & Security for 2 airlines as my work group managed the computer system, so I became very intimate with TSA regulations during the Secure Flight and CAPS security requirements for airlines, and I never have seen this requirement in the AOSSP (airline’s security program). That’s why I was taken aback when the flight attendants claimed it was a TSA regulation for US-bound aircraft.
I don’t mean to drag this thing out as it appears to have become a topic here, but too many flight attendants like to throw this around when its really NOT a rule.