As Lord Northcliffe discovered to his profit, doom is great for traffic. And in Gartner’s publication of its PC shipment forecast for 2009, the doom quotient is off the scale.

The analyst firm reckons the PC industry is set to suffer its sharpest unit decline in history, an 11.9% decline on last year. That, the research boffins point out, dwarfs the previous slump of 3.2% following the dotcom bubble incident of 2001.

Breaking that down: desk-based PCs are forecast to fall 31.9%, while mobile PC shipments will be up nine percent (2.7%, excluding mini-notebooks).

Mature markets are expected to shrink 13% while surprisingly, emerging markets will slip 10.4% – must be all those devious desktop recycling programs.

“The PC industry is facing extraordinary conditions as the global economy continues to weaken, users stretch PC lifetimes and PC suppliers grow increasingly cautious,” said George Shiffler, research director at Gartner.

“Slower GDP growth will generally weaken demand and slow new penetration, lengthening PC lifetimes will reduce replacements, and supplier caution will keep inventories at historic lows until confidence in a recovery eventually firms. The impact of reduced replacements will be especially acute in mature markets, where replacements are estimated to account for around 80 percent of shipments.”

From where IDC is sitting, however, the future doen’t seem so bleak. The IT market watcher predicted a total drop of global PC shipments of eight percent in the first half of the year, moving to “small positive growth” in the fourth quarter.” Overall, IDC tipped a total decline of 4.5% for the year, far more optimistic than Gartner.

“To be sure, the PC market is in for a bumpy ride,” said IDC director Loren Loverde. “Nevertheless, there are a number of reasons why the PC market will not fare dramatically worse in the current environment than it did in the 2001 recession. Pricing will become even more aggressive, and there will be further consolidation, but the PC industry will not go the way of the financial or auto industries in this cycle.”

Loren also pointed out that PCs are now “far more important” and “cost half” of what they did in 2000. The cheaper they become, he claimed, the more indispensable.

If the two leading IT sector watchers can’t agree on PC shipment patterns (which are often seen as a lead indicator for the whole of the IT industry), the only certainty going forward is still uncertainty.