Most enterprises needn't worry about the "viral" aspect of open-source licenses. Because most enterprises use software for internal purposes, rather than distribute it, they don't trigger the standard open-source requirement to contribute back derivative works. A recent Federal Computer Week article by John Moore does an admirable job of clarifying this.
There are, however, instances in which an enterprise might well trigger the contribution requirement of open-source licensing. If a company sold off a division to another company, complete with the servers running modified open-source software, this would likely trigger a "distribution" and might well affect the value of the deal.
For this and other instances, it's helpful to have a dual-licensing strategy. In this way, customers get all the benefits of open source, especially the ability to view and modify source code to suit their particular needs, without the obligation to contribute back derivative works.
Unfortunately, this perpetuates the problem that Jim Whitehurst of Red Hat has been highlighting: the more software created in isolation, the greater the industry's inefficiency and the higher the cost of software. Dual-licensing doesn't solve this problem. It is, however, a good way to help guide enterprises into open source on comforting terms.