Our customers tell us they are increasingly managing multiple websites. They typically want a platform that recognizes their (inevitably) unique approaches to centralization and distributed management. Many Web CMS tools can now "clone" websites, but this usually entails just copying a set of content, structural elements, vocabularies, and templates -- it does not address the problem of ongoing management of those assets.
Ongoing multi-site management typically requires some sort of object-oriented structure where you have central components that may or may not yield local (i.e., website-specific) derivatives or variants, depending on your rules. If you make a change to your standard corporate footer, you may want that to cascade down to all your web properties; if you change a navigation element on your "main" public site, you may not want that change inherited by all your other microsites.
As Web CMS Report readers know, there's a fairly clean divide in the industry between vendors who can do this and those who can't. Ironically, this is one of the many areas where most of the so-called "enterprise" tier vendors remain a year or more behind their more focused competitors.
Note, however, that there are few standard industry norms about how to organize multi-site management. Some products (e.g., Mediasurface Morello) allow you to maintain a central store of template, content, workflow, and other components separate from the actual website representations, and then you can build sites by drawing from this palette. I suspect this approach works well for media companies in particular, who want to share information across multiple properties without encumbering them too tightly from a structural standpoint.
Other tools assume a kind of "master" website, from which you can derive and manage local variants. This is particularly handy for multinational corporations who need to balance global and local communications across multiple properties under a single brand. The way that SDL Tridion controls variants is quite advanced in this regard, since you can create a master domain above your web properties that isn't actually a working website (i.e., your "main" website becomes a child as well). Of course, with power comes complexity, and Tridion customers sometimes report that it takes them some time and effort to fully grasp the concepts here.
But the bigger issue, as always, is one of governance. What kind of specific relationships do you want to create among your web properties? Issuing an RFP that just says "must integrate multiple websites" leaves you open to selecting a tool that doesn't let you standardize and experiment in the ways you want. We'll be watching this much more closely in the coming months...