I think the Gartner guys are talking more about custom development of, say industry or line of business specific applications. These applications may be built using open source components (Apache HTTP, AXIS etc). And enterprises will surely begin to increasingly choose open source applications like DimDim and Alfresco over WebEx and Documentum.
But when it comes to writing an application that allocates shipments into trucks based on volume/weight/destination etc., you’re going to probably write that app yourself. I think that the Gartner analysts are saying that when you do write that application, make sure you’re reusing functionality that you’ve developed in-house, or functionality that is being exposed to you from other parts of your supply/value chain, likely via web services.
It is possible that industry groups will realize that it makes more sense to write the foundation or a framework for an industry-specific applications jointly (like this example in the Healthcare industry), and use a BSD-based license that still enables each individual company to make the customizations appropriate for their individual internal processes, and to gain a competitive advantage.
But in either case, I agree with Matt that more open source usage is coming down the pipe ;-)