We have developed a large HR portal with GWT. The look and feel of this application can be customized for different deployments. To do this we use fragments of HTML to generate parts of the GUI i.e. bits of HTML are sent to the client in DTOs and then stuffed into HTML widgets. This approach works well for mastheads, logos, menus and so on.
Other things (e.g. capture forms) are generated using normal GWT code.
We use "pages" (different history tokens identifying where you are) as this makes it possible for users to use bookmarks in a meaningful way. We also generate links to different parts of our system in emails and so on.
Our application is composed of a lot of "higher level" widgets we call "Portlets" arranged into "pages" defined in XML. Again this makes it possible to customize the functionality for a given installation.
All this is done using a framework (GWT Portlets) that we have published as open source.