I noticed in the 2008 book it's used in the sample for the very first class built (Project) but the v4 sample project tracker doesn't use DataObjectField anywhere and I can find no mention of it in the change list.
The DataObjectField attribute applies only to ASP.NET Web Forms, and even there its utility is limited. If you are building a Web Forms UI you might still use this attribute if you find it helpful.
Since Web Forms is now just one of many UI types I've stopped putting that attribute into my samples, as it just causes confusion and clutter for most people.
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