Defying the global economic doldrums, numbers released today by the American Institute of Architects show that last month brought an unexpected jolt of hope for design firms, with billings for the hard-bitten industry breaking into positive territory after more than two years of declines.

In September, the monthly billings index surpassed the all-important 50 mark for the first time since January 2008, reaching a reading of 50.4. Even more surprising, inquiries for new projects shot up to 62.3 from 54.6, hitting their highest point since mid-2007. (Any score above 50 indicates an increase in billings or inquiries, and below 50 a decrease.)

The news consolidates four straight months of increases for the billings index, stoking hope that better times may finally be on the horizon. Still, given the tentative nature of the current recovery, the bump in billings must be viewed with caution.

“This is certainly encouraging news, but we will need to see consistent improvement over the next few months in order to feel comfortable about the state of the design and construction industry,” AIA Chief Economist Kermit Baker said in a release. “While there has been increasing demand for design services, it is happening at a slow rate and there continue to be other obstacles that are preventing a more accelerated recovery."

However, Baker added, "the strong upturn in design activity in the commercial and industrial sector certainly suggests that this upturn can possibly be sustained.”

Regionally, the Northeast continued its third month of positive growth, rising substantially to 56.7 from 50.9 in August. The Midwest and South both rose modestly, to 51.0 and 47.0 respectively, while the ever-suffering West slipped back a point to 44.5.

Among the sectors, commercial and industrial work surged ahead 6 points in September to 56.3, its fifth straight month above 50. The other three sectors fared less impressively: Institutional work increased to 47.9 from 46 in August, multifamily residential remained essentially flat at 47.0, and mixed practice rose slightly to 44.2.

The broader economic picture indicates that good times are still some ways away, with no major changes since last month’s sluggish outlook. Private housing starts were up a negligible 0.3 percent in September, according to a federal report released yesterday, while single-family building permits nudged up 0.5 percent. And the Federal Reserve’s Beige Book last month cited both “continued growth” and “widespread signs of a deceleration” in economic activity—mixed messages that will doubtless be familiar to architects as the industry continues its long battle back from the brink.