I’ve been hearing this a lot in the news, this year, meaning “a line that must not be crossed” or perhaps “a line whose crossing will instigate severe action”.

“My red line is Iran may not have a nuclear weapon,” Romney told ABC News. “It is inappropriate for them to have the capacity to terrorize the world. Iran as a nuclear nation is unacceptable to the United States of America.”

Is this a fairly new buzzphrase? I haven’t really noticed it before. Has it picked up in popularity this year?

It’s been around for a long time. The OED has a good account of the literal red line, coming from engine gauges, back to 1941, but doesn’t document the metaphorical usage well.

Related to the “marked limit on a gauge” sense is the verb usage: “The engine redlines at 6000 rpm”.