The new forecast calls for companies and governments to spend a combined $3.8 trillion on information technology, which would amount to growth of 3.7 percent from 2011. The previous forecast had called for growth of 4.6 percent.

For perspective, the difference on a dollar basis is about $100 billion, which is certainly real money, but when you consider the various puts and takes affecting the projected spend, it makes a certain amount of sense.

Gartner says that all four of the major technology sectors it tracks — computing hardware, enterprise software, IT services, and telecom equipment and services — will see their growth rates slow this year.

You can probably guess why: The uncertain global economy, the euro zone sovereign debt crisis and the disruptions on the hardware supply chain from last year’s flooding in Thailand on hard-drive production have all teamed up to perform a triple whammy on the tech sector. The Thailand problem will probably last until well into 2013, Gartner’s Richard Gordon says in a statement, echoing what Seagate CEO Steve Luczo told AllThingsD in an interview in November.

Telecom equipment spending will probably suffer the least, Gartner says. Sales in that sector will grow by nearly 7 percent to $475 billion, followed by the enterprise software market, which will grow by 6.4 percent to $285 billion. The chart at the right, which I screengrabbed from Gartner’s handout, breaks down the revised outlook by each sector versus what the previous growth outlook had been.

Gartner also trimmed its average annual growth projection for IT spending through 2015. It now expects spending to grow by about 5 percent on average, down only slightly from 5.4 percent, but in the wider scope of a few trillion dollars, a fractional change still amounts to hundreds of billions of dollars.

The good news is that Gartner has revised its outlook upward. Companies and governments will spend $3.6 trillion on IT this year, which is more than the prior $3.4 trillion forecast, amounting to growth of 5.1 percent. Sounds great, right?

Yes, but it’s complicated, especially from the U.S. point of view. The weak dollar makes the figures look a little better than they are. In 2010, Gartner says, IT spending grew 2.2 percent, but more than half of that–1.6 percent–can be attributed to the devaluation of the dollar against other currencies. Companies and governments spending other currencies can get more dollars for their money, and so this tends to inflate the appearance of growth, Gartner’s Richard Gordon told me.

A weak dollar is generally good news for U.S. companies that do a lot of global business. U.S. products and services look more attractive to non-U.S. buyers. But in cases like this, U.S. companies end up paying more for items that get imported and for raw materials.

That’s not to say there isn’t actual growth. Gartner says spending is picking up fastest on telecom equipment, with computing hardware and enterprise software following close behind.

Spending on discretionary items like IT services and consulting is coming back the slowest. When the economic crisis hit in late 2008 and early 2009 these were the first items on the chopping block, and spending on them is only now beginning to make a comeback.

No area of technology will be immune to the decline. Hardest hit: the computer hardware sector, which is expected to see spending fall 15 percent to $324.3 billion. Seems even the promise of government stimulus packages won’t be enough to offset this ugly near-term outlook. Said Gordon, “Economic conditions have continued to erode business confidence in all regions. There is a continued general sense of uncertainty in the market and a lack of clarity of actual amount of toxic debt out there. IT organizations will look for ways to shift spending from capital expenditures to operational efficiencies.”