Strong U.S. data, dollar down.

That was the rule for much of 2011. Indeed, that negative correlation between the greenback and U.S. numbers was the anchor of the so-called risk trade.

The transmission mechanism was simple enough: any sign of a stronger U.S. economy merely encouraged investors to take positions in riskier assets. Weakness had them scurrying back to the shelter of the dollar, the broadest shield they'd ever known, no matter that it was weakness in the dollar's home economy.

This in turn meant that all risky assets stood or fell together, regardless of their own fundamentals.

However, this pattern has not held of late. Since the final quarter of 2011, the correlation between the dollar and U.S. data has turned positive while the 12-month rolling link between the S&P 500 and EUR/USD has tumbled to 0.30 from record highs close to 0.9.

But as Monument Securities chief economist Stephen Lewis wrote on Tuesday, "the expectations that drive financial markets in the early days of January rarely survive far into the year."

Sure enough, analysts have plenty to say about the durability of this new, pro-cyclical dollar and, by extension, the future of the risk trade and those weakening correlations. Few seem to think they can last.

Indeed those at Standard Chartered think we could soon be back in an environment of relative dollar strength, even if the U.S. numbers turn sour.

They point out that expectations for more quantitative easing from the Fed remain in place, and that the dollar tends to weaken only after it's delivered, not while we wait.

Then there's the euro zone, where the euro remains extremely vulnerable to risk aversion and is forecast to slide further against the dollar.

If the bank is right we could see the U.S currency go counter-cyclical again; which might just put the old year's highly correlated risk trade back on again.