You can still create multiple layouts for one mxt. I do this all of the time. The main difference is that you save your layout as a mxd instead of mxt. Just save the mxd with the appropriate layout in: C:\Program Files (x86)\ArcGIS\Desktop 10.0\MapTemplates (Windows 7, I'm sure its very similar in previous versions of Windows) Then when you click the Change Layout button on the Layout toolbar it will still bring up the same dialog from previous versions. I think you can actually save them other places but this will assure that it is in the same place as all the built in templates. You just create a new folder and it should create a new tab in the dialog. Also, Data Driven pages is not designed for this, they are for creating mapbooks. I'm not sure why they couldn't just call the feature Mapbook but I'm sure Esri has their reasons.
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