What is your threshold for landing (or departing) downwind? Will you land
knowing you have a 5 knot tailwind? 10 knots? At what point will you
"flip" the pattern to takeoff or land into the wind?

Me? I'm a 5 knot kind of guy (but I fly a taildragger)...

I landed at 3 different fields today where the established pattern resulted
in a landing with a quartering 7 knot (or more) tailwind, with some mild
gusts thrown in for good measure.

At the first field, I was careless enough to assume the traffic was using
the runway facing the wind. The downwind landing came as a bit of a
surprise, then I looked at the wind sock - oops...

At the other two fields, I announced my position and intentions and waited
(360's on the downwind) until I could land into the wind without interfering
with aircraft already in the pattern.

And what is the correct radio phraseology to say: "Fellas, it is time to
reverse the pattern - you're taking off and landing downwind.." ?