Somewhere out there lies a trigger point for Federal Reserve rate hikes.

While that policy tightening may not come for many months or even years, economists still believe the key variable is the interplay between employment and inflation.

That interplay, most economists believe, has changed in the wake of the worst recession in generations. Structural changes in the economy mean higher rates of unemployment will be the new normal. As a result, inflation could start to well up from levels of joblessness that until recently had been benign for price pressures.

At the heart of the issue is what economists call NAIRU, or Non-Accelerating Inflation Rate of Unemployment. It’s essentially the lowest level of unemployment that, if breached, will lead to rising inflationary pressure.

NAIRU is tough to pin down, but even so, the concept has currency with many economists and Fed policy makers. Central bankers generally deal with the issue in a roundabout way, by talking about how much excess capacity the economy has. They also offer assessments of the output gap, or the difference between where the economy’s potential growth is and where activity gains actually are. The more negative this gap is, the less inflation should arise.

In this way NAIRU bleeds back into the debate over what the Federal Reserve will do with monetary policy. Economists are increasingly gravitating to the view NAIRU, roughly speaking, is higher than it once was. That means the Fed has less wiggle room: higher levels of unemployment may already be able to generate the sort of inflation central bankers need to counter with tighter monetary policy.

The Fed itself doesn’t make available a NAIRU forecast. The Congressional Budget Office, as part of making its own economic projections, does and puts the measure at 5% through 2020 — about where it’s been for the last couple of decades.

Deutsche Bank economists believe NAIRU is on the rise. In a research note the bank said they believe the measure was at 5.6% in 2009 and could hit 6% this year. They tie the rise to the long-term nature of unemployment seen in the wake of the recession. Also, some industries, like the auto sectors, have cut back capacity for years to come. Extended unemployment insurance benefits also play a role, the economists said.

UBS economist Drew Matus concurs that NAIRU is “drifting a little higher” relative to the past, pegging it at around 6% to 6.5%.

Economists’ evolving view on NAIRU has at best a minimal role to play in the monetary policy game right now. May’s 9.7% unemployment rate is very high and a long way from anything that will drive the Fed to end its near-zero% interest rate policy. Indeed, this gap is a fundamental reason why so many forecasters believe the Fed can make it into 2011 without acting.

That said, NAIRU isn’t a problem-free concept. Calculating it requires an analyst to make assumptions about what the economy’s potential growth rate is. In a real sense, NAIRU, like other rule-based policy tools, are rooted in things that aren’t actually measurable, at least in something close to real time.

“You might as well ask what is the tooth fairy’s favorite color or what sound does a rainbow make,” said Ian Shepherdson, of High Frequency Economics. The NAIRU concept asserts the economy “is permanently highly unstable” and is always a step away from a highly unstable inflation environment, the economist said, adding “there has been no evidence to support this idea in the past 30 years.”